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L_0744 | wave interactions and interference | T_3730 | An echo is an example of wave reflection. Reflection occurs when waves bounce back from a barrier they cannot pass through. Reflection can happen with any type of waves, not just sound waves. For example, Figure 19.15 shows the reflection of ocean waves off a rocky coast. Light waves can also be reflected. In fact, thats how we see most objects. Light from a light source, such as the sun or a light bulb, shines on the object and some of the light is reflected. When the reflected light enters our eyes, we can see the object. Reflected waves have the same speed and frequency as the original waves before they were reflected. However, the direction of the reflected waves is different. When waves strike an obstacle head on, the reflected waves bounce straight back in the direction they came from. When waves strike an obstacle at any other angle, they bounce back at the same angle but in a different direction. This is illustrated in Figure 19.16. | text | null |
L_0744 | wave interactions and interference | T_3730 | An echo is an example of wave reflection. Reflection occurs when waves bounce back from a barrier they cannot pass through. Reflection can happen with any type of waves, not just sound waves. For example, Figure 19.15 shows the reflection of ocean waves off a rocky coast. Light waves can also be reflected. In fact, thats how we see most objects. Light from a light source, such as the sun or a light bulb, shines on the object and some of the light is reflected. When the reflected light enters our eyes, we can see the object. Reflected waves have the same speed and frequency as the original waves before they were reflected. However, the direction of the reflected waves is different. When waves strike an obstacle head on, the reflected waves bounce straight back in the direction they came from. When waves strike an obstacle at any other angle, they bounce back at the same angle but in a different direction. This is illustrated in Figure 19.16. | text | null |
L_0744 | wave interactions and interference | T_3731 | Refraction is another way that waves interact with matter. Refraction occurs when waves bend as they enter a new medium at an angle. You can see an example of refraction in Figure 19.17. Light bends when it passes from air to water. The bending of the light causes the pencil to appear broken. Why do waves bend as they enter a new medium? Waves usually travel at different speeds in different media. For example, light travels more slowly in water than air. This causes it to refract when it passes from air to water. | text | null |
L_0744 | wave interactions and interference | T_3731 | Refraction is another way that waves interact with matter. Refraction occurs when waves bend as they enter a new medium at an angle. You can see an example of refraction in Figure 19.17. Light bends when it passes from air to water. The bending of the light causes the pencil to appear broken. Why do waves bend as they enter a new medium? Waves usually travel at different speeds in different media. For example, light travels more slowly in water than air. This causes it to refract when it passes from air to water. | text | null |
L_0744 | wave interactions and interference | T_3732 | Did you ever notice that when youre walking down a street, you can hear sounds around the corners of buildings? Figure 19.18 shows why this happens. As you can see from the figure, sound waves spread out and travel around obstacles. This is called diffraction. It also occurs when waves pass through an opening in an obstacle. All waves may be diffracted, but it is more pronounced in some types of waves than others. For example, sound waves bend around corners much more than light does. Thats why you can hear but not see around corners. For a given type of waves, such as sound waves, how much the waves diffract depends on two factors: the size of the obstacle or opening in the obstacle and the wavelength. This is illustrated in Figure 19.19. Diffraction is minor if the length of the obstacle or opening is greater than the wavelength. Diffraction is major if the length of the obstacle or opening is less than the wavelength. | text | null |
L_0744 | wave interactions and interference | T_3732 | Did you ever notice that when youre walking down a street, you can hear sounds around the corners of buildings? Figure 19.18 shows why this happens. As you can see from the figure, sound waves spread out and travel around obstacles. This is called diffraction. It also occurs when waves pass through an opening in an obstacle. All waves may be diffracted, but it is more pronounced in some types of waves than others. For example, sound waves bend around corners much more than light does. Thats why you can hear but not see around corners. For a given type of waves, such as sound waves, how much the waves diffract depends on two factors: the size of the obstacle or opening in the obstacle and the wavelength. This is illustrated in Figure 19.19. Diffraction is minor if the length of the obstacle or opening is greater than the wavelength. Diffraction is major if the length of the obstacle or opening is less than the wavelength. | text | null |
L_0744 | wave interactions and interference | T_3733 | Waves interact not only with matter in the ways described above. Waves also interact with other waves. This is called wave interference. Wave interference may occur when two waves that are traveling in opposite directions meet. The two waves pass through each other, and this affects their amplitude. How amplitude is affected depends on the type of interference. Interference can be constructive or destructive. | text | null |
L_0744 | wave interactions and interference | T_3734 | Constructive interference occurs when the crests of one wave overlap the crests of the other wave. This is illustrated in Figure 19.20. As the waves pass through each other, the crests combine to produce a wave with greater amplitude. You can see an animation of constructive interference at this URL: http://phys23p.sl.psu.edu/phys_anim/waves/em | text | null |
L_0744 | wave interactions and interference | T_3735 | Destructive interference occurs when the crests of one wave overlap the troughs of another wave. This is illustrated in Figure 19.21. As the waves pass through each other, the crests and troughs cancel each other out to produce a wave with less amplitude. You can see an animation of destructive interference at this URL: http://phys23p.sl.psu.ed | text | null |
L_0744 | wave interactions and interference | T_3736 | When a wave is reflected straight back from an obstacle, the reflected wave interferes with the original wave and creates a standing wave. This is a wave that appears to be standing still. A standing wave occurs because of a combination of constructive and destructive interference between a wave and its reflected wave. You can see animations of standing waves at the URLs below. http://skullsinthestars.com/2008/05/04/classic-science-paper-otto-wieners-experiment-1890/ Its easy to generate a standing wave in a rope by tying one end to a fixed object and moving the other end up and down. When waves reach the fixed object, they are reflected back. The original wave and the reflected wave interfere to produce a standing wave. Try it yourself and see if the wave appears to stand still. | text | null |
L_0748 | characteristics of sound | T_3770 | Why does a tree make sound when it crashes to the ground? How does the sound reach peoples ears if they happen to be in the forest? And in general, how do sounds get started, and how do they travel? Keep reading to find out. | text | null |
L_0748 | characteristics of sound | T_3771 | All sounds begin with vibrating matter. It could be the ground vibrating when a tree comes crashing down. Or it could be guitar strings vibrating when they are plucked. You can see a guitar string vibrating in Figure 20.2. The vibrating string repeatedly pushes against the air particles next to it. The pressure of the vibrating string causes these air particles to vibrate. The air particles alternately push together and spread apart. This starts waves of vibrations that travel through the air in all directions away from the strings. The vibrations pass through the air as longitudinal waves, with individual air particles vibrating back and forth in the same direction that the waves travel. You can see an animation of sound waves moving through air at this URL: | text | null |
L_0748 | characteristics of sound | T_3772 | Sound waves are mechanical waves, so they can travel only though matter and not through empty space. This was demonstrated in the 1600s by a scientist named Robert Boyle. Boyle placed a ticking clock in a sealed glass jar. The clock could be heard ticking through the air and glass of the jar. Then Boyle pumped the air out of the jar. The clock was still running, but the ticking could no longer be heard. Thats because the sound couldnt travel away from the clock without air particles to pass the sound energy along. You can see an online demonstration of the same experimentwith a modern twistat this URL: (4:06). MEDIA Click image to the left or use the URL below. URL: Sound waves can travel through many different kinds of matter. Most of the sounds we hear travel through air, but sounds can also travel through liquids such as water and solids such as glass and metal. If you swim underwater or even submerge your ears in bathwater any sounds you hear have traveled to your ears through water. You can tell that sounds travel through glass and other solids because you can hear loud outdoor sounds such as sirens through closed windows and doors. | text | null |
L_0748 | characteristics of sound | T_3773 | Sound has certain characteristic properties because of the way sound energy travels in waves. Properties of sound include speed, loudness, and pitch. | text | null |
L_0748 | characteristics of sound | T_3774 | The speed of sound is the distance that sound waves travel in a given amount of time. You probably already know that sound travels more slowly than light. Thats why you usually see the flash of lightning before you hear the boom of thunder. However, the speed of sound isnt constant. It varies depending on the medium of the sound waves. Table 20.1 lists the speed of sound in several different media. Generally, sound waves travel fastest through solids and slowest through gases. Thats because the particles of solids are close together and can quickly pass the energy of vibrations to nearby particles. You can explore the speed of sound in different media at this URL: Medium (20C) Air Water Wood Glass Aluminum Speed of Sound Waves (m/s) 343 1437 3850 4540 6320 The speed of sound also depends on the temperature of the medium. For a given medium such as air, sound has a slower speed at lower temperatures. You can compare the speed of sound in air at different temperatures in Table transfer the energy of the sound waves. The amount of water vapor in the air affects the speed of sound as well. Do you think sound travels faster or slower when the air contains more water vapor? (Hint: Compare the speed of sound in water and air in Table 20.1.) Temperature of Air 0C 20C 100C Speed of Sound (m/s) 331 343 386 KQED: Speed of Sound Along with cable cars and seagulls, the Golden Gate Bridge foghorn is one of San Franciscos most iconic sounds. But did you know that if you hear that foghorn off in the distance, you can calculate how many miles you are from the bridge? Using the Speed of Sound exhibit at the Outdoor Exploratorium at Fort Mason, Shawn Lani shows us how sound perception is affected by distance. For more information on the speed of sound, see http://science.kqed. MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0748 | characteristics of sound | T_3775 | A friend whispers to you in class in a voice so soft that you have to lean very close to hear what hes saying. Later that day, your friend shouts to you across the football field. Now his voice is loud enough for you to hear him clearly even though hes many meters away. Obviously, sounds can vary in loudness. Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity of sound. Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel (dB). You can see typical decibel levels of several different sounds in Figure 20.3. As decibel levels get higher, sound waves have greater intensity and sounds are louder. For every 10-decibel increase in the intensity of sound, loudness is 10 times greater. Therefore, a 30-decibel "quiet" room is 10 times louder than a 20-decibel whisper, and a 40- decibel light rainfall is 100 times louder than a 20-decibel whisper. How much louder than a 20-decibel whisper is the 60-decibel sound of a vacuum cleaner? The intensity of sound waves determines the loudness of sounds, but what determines intensity? Intensity is a function of two factors: the amplitude of the sound waves and how far they have traveled from the source of the sound. Remember that sound waves start at a source of vibrations and spread out from the source in all directions. The farther the sound waves travel away from the source, the more spread out their energy becomes. This is illustrated in Figure 20.4. The decrease in intensity with distance from a sound source explains why even loud sounds fade away as you move farther from the source. It also explains why low-amplitude sounds can be heard only over short distances. For a video demonstration of the amplitude and loudness of sounds, go to this URL: interactive animation at this URL: | text | null |
L_0748 | characteristics of sound | T_3775 | A friend whispers to you in class in a voice so soft that you have to lean very close to hear what hes saying. Later that day, your friend shouts to you across the football field. Now his voice is loud enough for you to hear him clearly even though hes many meters away. Obviously, sounds can vary in loudness. Loudness refers to how loud or soft a sound seems to a listener. The loudness of sound is determined, in turn, by the intensity of sound. Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel (dB). You can see typical decibel levels of several different sounds in Figure 20.3. As decibel levels get higher, sound waves have greater intensity and sounds are louder. For every 10-decibel increase in the intensity of sound, loudness is 10 times greater. Therefore, a 30-decibel "quiet" room is 10 times louder than a 20-decibel whisper, and a 40- decibel light rainfall is 100 times louder than a 20-decibel whisper. How much louder than a 20-decibel whisper is the 60-decibel sound of a vacuum cleaner? The intensity of sound waves determines the loudness of sounds, but what determines intensity? Intensity is a function of two factors: the amplitude of the sound waves and how far they have traveled from the source of the sound. Remember that sound waves start at a source of vibrations and spread out from the source in all directions. The farther the sound waves travel away from the source, the more spread out their energy becomes. This is illustrated in Figure 20.4. The decrease in intensity with distance from a sound source explains why even loud sounds fade away as you move farther from the source. It also explains why low-amplitude sounds can be heard only over short distances. For a video demonstration of the amplitude and loudness of sounds, go to this URL: interactive animation at this URL: | text | null |
L_0748 | characteristics of sound | T_3776 | A marching band is parading down the street. You can hear it coming from several blocks away. When the different instruments finally pass by you, their distinctive sounds can be heard. The tiny piccolos trill their bird-like high notes, and the big tubas rumble out their booming bass notes (see Figure 20.5). Clearly, some sounds are higher or lower than others. But do you know why? How high or low a sound seems to a listener is its pitch. Pitch, in turn, depends on the frequency of sound waves. Recall that the frequency of waves is the number of waves that pass a fixed point in a given amount of time. High-pitched sounds, like the sounds of a piccolo, have high-frequency waves. Low-pitched sounds, like the sounds of a tuba, have low-frequency waves. For a video demonstration of frequency and pitch, go to this URL: (3:20). MEDIA Click image to the left or use the URL below. URL: To explore an interactive animation of sound wave frequency, go to this URL: The frequency of sound waves is measured in hertz (Hz), or the number of waves that pass a fixed point in a second. Human beings can normally hear sounds with a frequency between about 20 Hz and 20,000 Hz. Sounds with frequencies below 20 hertz are called infrasound. Sounds with frequencies above 20,000 hertz are called ultrasound. Some other animals can hear sounds in the ultrasound range. For example, dogs can hear sounds with frequencies as high as 50,000 Hz. You may have seen special whistles that dogs but not people can hear. The whistles produce a sound with a frequency too high for the human ear to detect. Other animals can hear even higher-frequency sounds. Bats, for example, can hear sounds with frequencies higher than 100,000 Hz. | text | null |
L_0748 | characteristics of sound | T_3777 | Look at the police car in Figure 20.6. The sound waves from its siren travel outward in all directions. Because the car is racing forward (toward the right), the sound waves get bunched up in front of the car and spread out behind it. As the car approaches the person on the right (position B), the sound waves get closer and closer together. In other words, they have a higher frequency. This makes the siren sound higher in pitch. After the car speeds by the person on the left (position A), the sound waves get more and more spread out, so they have a lower frequency. This makes the siren sound lower in pitch. A change in the frequency of sound waves, relative to a stationary listener, when the source of the sound waves is moving is called the Doppler effect. Youve probably experienced the Doppler effect yourself. The next time a vehicle with a siren races by, listen for the change in pitch. For an online animation of the Doppler effect, go to the URL below. | text | null |
L_0749 | hearing sound | T_3778 | Figure 20.7 shows the three main parts of the ear: the outer, middle, and inner ear. It also shows the specific structures in each part. The roles of these structures in hearing are described below and in the animations at these URLS: (1:43) MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0749 | hearing sound | T_3779 | The outer ear includes the pinna, ear canal, and eardrum. The pinna is the only part of the ear that extends outward from the head. Its position and shape make it good at catching sound waves and funneling them into the ear canal. The ear canal is a tube that carries sound waves into the ear. The sound waves travel through the air inside the ear canal to the eardrum. The eardrum is like the head of a drum. Its a thin membrane stretched tight across the end of the ear canal. The eardrum vibrates when sound waves strike it, and it sends the vibrations on to the middle ear. | text | null |
L_0749 | hearing sound | T_3780 | The middle ear contains three tiny bones (ossicles) called the hammer, anvil, and stirrup. If you look at these bones in Figure 20.7, you might notice that they resemble the objects for which they are named. The three bones transmit vibrations from the eardrum to the inner ear. They also amplify the vibrations. The arrangement of the three bones allows them to work together as a lever that increases the amplitude of the waves as they pass to the inner ear. | text | null |
L_0749 | hearing sound | T_3781 | The stirrup passes the amplified sound waves to the inner ear through the oval window (see Figure 20.7). When the oval window vibrates, it causes the cochlea to vibrate as well. The cochlea is a shell-like structure that is full of fluid and lined with nerve cells called hair cells. Each hair cell has tiny hair-like projections, as you can see in Figure and this triggers electrical impulses. The electrical impulses travel to the brain through nerves. Only after the nerve impulses reach the brain do we hear the sound. | text | null |
L_0749 | hearing sound | T_3782 | All these structures of the ear must work well for normal hearing. Damage to any of them, through illness or injury, may cause hearing loss. Total hearing loss is called deafness. To learn more about hearing loss, watch the animation at this URL: (1:39). MEDIA Click image to the left or use the URL below. URL: Most adults experience at least some hearing loss as they get older. The most common cause is exposure to loud sounds, which damage hair cells. The louder a sound is, the less exposure is needed for damage to occur. Even a single brief exposure to a sound louder than 115 decibels can cause hearing loss. Figure 20.9 shows the relationship between loudness, exposure time, and hearing loss. | text | null |
L_0749 | hearing sound | T_3782 | All these structures of the ear must work well for normal hearing. Damage to any of them, through illness or injury, may cause hearing loss. Total hearing loss is called deafness. To learn more about hearing loss, watch the animation at this URL: (1:39). MEDIA Click image to the left or use the URL below. URL: Most adults experience at least some hearing loss as they get older. The most common cause is exposure to loud sounds, which damage hair cells. The louder a sound is, the less exposure is needed for damage to occur. Even a single brief exposure to a sound louder than 115 decibels can cause hearing loss. Figure 20.9 shows the relationship between loudness, exposure time, and hearing loss. | text | null |
L_0749 | hearing sound | T_3783 | Hearing loss caused by loud sounds is permanent. However, this type of hearing loss can be prevented by protecting the ears from loud sounds. | text | null |
L_0749 | hearing sound | T_3784 | People who work in jobs that expose them to loud sounds must wear hearing protectors. Examples include construc- tion workers who work around loud machinery for many hours each day (see Figure 20.10). But anyone exposed to loud sounds for longer than the permissible exposure time should wear hearing protectors. Many home and yard chores and even recreational activities are loud enough to cause hearing loss if people are exposed to them for very long. | text | null |
L_0749 | hearing sound | T_3785 | You can see two different types of hearing protectors in Figure 20.11. Earplugs are simple hearing protectors that just muffle sounds by partially blocking all sound waves from entering the ears. This type of hearing protector is suitable for lower noise levels, such as the noise of a lawnmower or snowmobile engine. Electronic ear protectors work differently. They identify high-amplitude sound waves and send sound waves through them in the opposite direction. This causes destructive interference with the waves, which reduces their amplitude to zero or nearly zero. This changes even the loudest sounds to just a soft hiss. Sounds that people need to hear, such as the voices of co-workers, are not interfered with in this way and may be amplified instead so they can be heard more clearly. This type of hearing protector is recommended for higher noise levels and situations where its important to be able to hear lower-decibel sounds. | text | null |
L_0750 | using sound | T_3786 | People have been using sound to make music for thousands of years. They have invented many different kinds of musical instruments for this purpose. Despite their diversity, however, musical instruments share certain similarities. All musical instruments create sound by causing matter to vibrate. The vibrations start sound waves moving through the air. Most musical instruments use resonance to amplify the sound waves and make the sounds louder. Resonance occurs when an object vibrates in response to sound waves of a certain frequency. In a musical instrument such as a guitar, the whole instrument and the air inside it may vibrate when a single string is plucked. This causes constructive interference with the sound waves, which increases their amplitude. Most musical instruments have a way of changing the frequency of the sound waves they produce. This changes the pitch of the sounds. There are three basic categories of musical instruments: percussion, wind, and stringed instruments. In Figure | text | null |
L_0750 | using sound | T_3787 | Researchers at Lawrence Berkeley National Laboratory are pioneering a new way to recover 100-year-old record- ings. Found on fragile wax cylinders and early lacquer records, the sounds reveal a rich acoustic heritage, including languages long lost. For more information on how to recover recordings, see http://science.kqed.org/quest/video/ MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0750 | using sound | T_3788 | Ultrasound has frequencies higher than the human ear can detect (higher than 20,000 hertz). Although we cant hear ultrasound, it is very useful. Uses include echolocation, sonar, and ultrasonography. | text | null |
L_0750 | using sound | T_3789 | Animals such as bats, whales, and dolphins send out ultrasound waves and use their echoes, or reflected waves, to identify the locations of objects they cannot see. This is called echolocation. Animals use echolocation to find prey and avoid running into objects in the dark. Figure 20.13 and the animation at the URL below show how a bat uses echolocation to locate insect prey. | text | null |
L_0750 | using sound | T_3790 | Sonar uses ultrasound in a way that is similar to echolocation. Sonar stands for sound navigation and ranging. It is used to locate underwater objects such as sunken ships or to determine how deep the water is. A sonar device is usually located on a boat at the surface of the water. The device is both a sender and a receiver (see Figure 20.14). It sends out ultrasound waves and detects reflected waves that bounce off underwater objects or the bottom of the water. If you watch the video at the URL below, you can see how sonar is used on a submarine. The distance to underwater objects or the bottom of the water can be calculated from the known speed of sound in water and the time it takes for the waves to travel to the object. The equation for the calculation is: Distance = Speed Time Assume, for example, that a sonar device on a ship sends an ultrasound wave to the bottom of the ocean. The speed of the sound through ocean water is 1437 m/s, and the wave travels to the bottom and back in 2 seconds. What is the distance from the surface to the bottom of the water? The sound wave travels to the bottom and back in 2 seconds, so it travels from the surface to the bottom in 1 second. Therefore, the distance from the surface to the bottom is: Distance = 1437 m/s 1 s = 1437 m You Try It! Problem: The sonar device on a ship sends an ultrasound wave to the bottom of the water at speed of 1437 m/s. The wave is reflected back to the device in 4 seconds. How deep is the water? | text | null |
L_0750 | using sound | T_3791 | Ultrasound can be used to "see" inside the human body. This use of ultrasound is called ultrasonography. Harmless ultrasound waves are sent inside the body, and the reflected waves are used to create an image on a screen. This technology is used to examine internal organs and unborn babies without risk to the patient. You can see an ultrasound image in Figure 20.15. You can see an animation showing how ultrasonography works at this URL: | text | null |
L_0750 | using sound | T_3792 | In this QUEST web extra, Stanford University astrophysicist Todd Hoeksema explains how solar sound waves are a vital ingredient to the science of helioseismology, in which the interior properties of the sun are probed by analyzing and tracking the surface sound waves that bounce into and out of the Sun. For more information on solar sound waves, see http://science.kqed.org/quest/video/web-extra-music-of-the-sun/ . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0751 | electromagnetic waves | T_3793 | An electromagnetic wave is a wave that consists of vibrating electric and magnetic fields. A familiar example will help you understand the fields that make up an electromagnetic wave. Think about a common bar magnet. It exerts magnetic force in an area surrounding it, called the magnetic field. You can see the magnetic field of a bar magnet in Figure 21.1. Because of this force field, a magnet can exert force on objects without touching them. They just have to be in its magnetic field. An electric field is similar to a magnetic field (see Figure 21.1). An electric field is an area of electrical force surrounding a charged particle. Like a magnetic field, an electric field can exert force on objects over a distance without actually touching them. | text | null |
L_0751 | electromagnetic waves | T_3794 | An electromagnetic wave begins when an electrically charged particle vibrates. This is illustrated in Figure 21.2. When a charged particle vibrates, it causes the electric field surrounding it to vibrate as well. A vibrating electric field, in turn, creates a vibrating magnetic field (you can learn how this happens in the chapter "Electromagnetism"). The two types of vibrating fields combine to create an electromagnetic wave. You can see an animation of an electromagnetic wave at this URL: (1:31). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0751 | electromagnetic waves | T_3795 | As you can see in Figure 21.2, the electric and magnetic fields that make up an electromagnetic wave occur are at right angles to each other. Both fields are also at right angles to the direction that the wave travels. Therefore, an electromagnetic wave is a transverse wave. | text | null |
L_0751 | electromagnetic waves | T_3796 | Unlike a mechanical transverse wave, which requires a medium, an electromagnetic transverse wave can travel through space without a medium. Waves traveling through a medium lose some energy to the medium. However, when an electromagnetic wave travels through space, no energy is lost, so the wave doesnt get weaker as it travels. However, the energy is "diluted" as it spreads out over an ever-larger area as it travels away from the source. This is similar to the way a sound wave spreads out and becomes less intense farther from the sound source. | text | null |
L_0751 | electromagnetic waves | T_3797 | Electromagnetic waves can travel through matter as well as across space. When they strike matter, they interact with it in the same ways that mechanical waves interact with matter. They may reflect (bounce back), refract (bend when traveling through different materials), or diffract (bend around objects). They may also be converted to other forms of energy. Microwaves are a familiar example. They are a type of electromagnetic wave that you can read about later on in this chapter, in the lesson "The Electromagnetic Spectrum." When microwaves strike food in a microwave oven, they are converted to thermal energy, which heats the food. | text | null |
L_0751 | electromagnetic waves | T_3798 | Electromagnetic radiation behaves like waves of energy most of the time, but sometimes it behaves like particles. As evidence accumulated for this dual nature of electromagnetic radiation, the famous physicist Albert Einstein developed a new theory about electromagnetic radiation, called the wave-particle theory. This theory explains how electromagnetic radiation can behave as both a wave and a particle. In brief, when an electron returns to a lower energy level, it is thought to give off a tiny "packet" of energy called a photon (see Figure 21.3). The amount of energy in a photon may vary. It depends on the frequency of electromagnetic radiation. The higher the frequency is, the more energy a photon has. | text | null |
L_0751 | electromagnetic waves | T_3799 | The most important source of electromagnetic radiation on Earth is the sun. Electromagnetic waves travel from the sun to Earth across space and provide virtually all the energy that supports life on our planet. Many other sources of electromagnetic waves that people use depend on technology. Radio waves, microwaves, and X rays are examples. We use these electromagnetic waves for communications, cooking, medicine, and many other purposes. Youll learn about all these types of electromagnetic waves in this chapters lesson on "The Electromagnetic Spectrum." | text | null |
L_0752 | properties of electromagnetic waves | T_3800 | All electromagnetic waves travel at the same speed through empty space. That speed, called the speed of light, is 300 million meters per second (3.0 108 m/s). Nothing else in the universe is known to travel this fast. If you could move that fast, you would be able to travel around Earth 7.5 times in just 1 second! The sun is about 150 million kilometers (93 million miles) from Earth, but it takes electromagnetic radiation only 8 minutes to reach Earth from the sun. Electromagnetic waves travel more slowly through a medium, and their speed may vary from one medium to another. For example, light travels more slowly through water than it does through air (see Figure 21.4). You can learn more about the speed of light at this URL: http://videos.howstuffworks.com/discovery/29407-assignme | text | null |
L_0752 | properties of electromagnetic waves | T_3801 | Although all electromagnetic waves travel at the same speed, they may differ in their wavelength and frequency. | text | null |
L_0752 | properties of electromagnetic waves | T_3802 | Wavelength and frequency are defined in the same way for electromagnetic waves as they are for mechanical waves. Both properties are illustrated in Figure 21.5. Wavelength is the distance between corresponding points of adjacent waves. Wavelengths of electromagnetic waves range from many kilometers to a tiny fraction of a millimeter. Frequency is the number of waves that pass a fixed point in a given amount of time. Frequencies of electro- magnetic waves range from thousands to trillions of waves per second. Higher frequency waves have greater energy. | text | null |
L_0752 | properties of electromagnetic waves | T_3803 | The speed of a wave is a product of its wavelength and frequency. Because all electromagnetic waves travel at the same speed through space, a wave with a shorter wavelength must have a higher frequency, and vice versa. This relationship is represented by the equation: Speed = Wavelength Frequency The equation for wave speed can be rewritten as: Frequency = Speed Speed or Wavelength = Wavelength Frequency Therefore, if either wavelength or frequency is known, the missing value can be calculated. Consider an electromag- netic wave that has a wavelength of 3 meters. Its speed, like the speed of all electromagnetic waves, is 3.0 108 meters per second. Its frequency can be found by substituting these values into the frequency equation: Frequency = 3.0 108 m/s = 1.0 108 waves/s, or 1.0 108 hertz (Hz) 3.0 m You Try It! Problem: What is the wavelength of an electromagnetic wave that has a frequency of 3.0 108 hertz? For more practice calculating the frequency and wavelength of electromagnetic waves, go to these URLs: | text | null |
L_0753 | the electromagnetic spectrum | T_3804 | Electromagnetic radiation occurs in waves of different wavelengths and frequencies. Infrared light and visible light make up just a small part of the full range of electromagnetic radiation, which is called the electromagnetic spectrum. The electromagnetic spectrum is summarized in the diagram in Figure 21.7. On the far left of the diagram are radio waves, which include microwaves. They have the longest wavelengths and lowest frequencies of all electromagnetic waves. They also have the least amount of energy. On the far right are X rays and gamma rays. The have the shortest wavelengths and highest frequencies of all electromagnetic waves. They also have the greatest amount of energy. Between these two extremes, wavelength, frequency, and energy change continuously from one side of the spectrum to the other. Waves in this middle section of the electromagnetic spectrum are commonly called light. As you will read below, the properties of electromagnetic waves influence how the different waves behave and how they can be used. | text | null |
L_0753 | the electromagnetic spectrum | T_3804 | Electromagnetic radiation occurs in waves of different wavelengths and frequencies. Infrared light and visible light make up just a small part of the full range of electromagnetic radiation, which is called the electromagnetic spectrum. The electromagnetic spectrum is summarized in the diagram in Figure 21.7. On the far left of the diagram are radio waves, which include microwaves. They have the longest wavelengths and lowest frequencies of all electromagnetic waves. They also have the least amount of energy. On the far right are X rays and gamma rays. The have the shortest wavelengths and highest frequencies of all electromagnetic waves. They also have the greatest amount of energy. Between these two extremes, wavelength, frequency, and energy change continuously from one side of the spectrum to the other. Waves in this middle section of the electromagnetic spectrum are commonly called light. As you will read below, the properties of electromagnetic waves influence how the different waves behave and how they can be used. | text | null |
L_0753 | the electromagnetic spectrum | T_3805 | Radio waves are the broad range of electromagnetic waves with the longest wavelengths and lowest frequencies. In Figure 21.7, you can see that the wavelength of radio waves may be longer than a soccer field. With their low frequencies, radio waves have the least energy of electromagnetic waves, but they still are extremely useful. They are used for radio and television broadcasts, microwave ovens, cell phone transmissions, and radar. You can learn more about radio waves, including how they were discovered, at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0753 | the electromagnetic spectrum | T_3806 | In radio broadcasts, sounds are encoded in radio waves that are sent out through the atmosphere from a radio tower. A receiver detects the radio waves and changes them back to sounds. Youve probably listened to both AM and FM radio stations. How sounds are encoded in radio waves differs between AM and FM broadcasts. AM stands for amplitude modulation. In AM broadcasts, sound signals are encoded by changing the amplitude of radio waves. AM broadcasts use longerwavelength radio waves than FM broadcasts. Because of their longer wavelengths, AM radio waves reflect off a layer of the upper atmosphere called the ionosphere. You can see how this happens in Figure 21.8. This allows AM radio waves to reach radio receivers that are very far away from the radio tower. FM stands for frequency modulation. In FM broadcasts, sound signals are encoded by changing the frequency of radio waves. Frequency modulation allows FM waves to encode more information than does amplitude modulation, so FM broadcasts usually sound clearer than AM broadcasts. However, because of their shorter wavelength, FM waves do not reflect off the ionosphere. Instead, they pass right through it and out into space (see Figure 21.8). As a result, FM waves cannot reach very distant receivers. | text | null |
L_0753 | the electromagnetic spectrum | T_3807 | Television broadcasts also use radio waves. Sounds are encoded with frequency modulation, and pictures are encoded with amplitude modulation. The encoded radio waves are broadcast from a TV tower like the one in Figure 21.9. When the waves are received by television sets, they are decoded and changed back to sounds and pictures. | text | null |
L_0753 | the electromagnetic spectrum | T_3808 | The shortest wavelength, highest frequency radio waves are called microwaves (see Figure 21.7). Microwaves have more energy than other radio waves. Thats why they are useful for heating food in microwave ovens. Microwaves have other important uses as well, including cell phone transmissions and radar, which is a device for determining the presence and location of an object by measuring the time for the echo of a radio wave to return from it and the direction from which it returns. These uses are described in Figure 21.10. You can learn more about microwaves and their uses in the video at this URL: (3:23). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0753 | the electromagnetic spectrum | T_3809 | Mid-wavelength electromagnetic waves are commonly called light. This range of electromagnetic waves has shorter wavelengths and higher frequencies than radio waves, but not as short and high as X rays and gamma rays. Light includes visible light, infrared light, and ultraviolet light. If you look back at Figure 21.7, you can see where these different types of light waves fall in the electromagnetic spectrum. | text | null |
L_0753 | the electromagnetic spectrum | T_3810 | The only light that people can see is called visible light. It refers to a very narrow range of wavelengths in the electromagnetic spectrum that falls between infrared light and ultraviolet light. Within the visible range, we see light of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light, which has the shortest wavelength. You can see the spectrum of colors of visible light in Figure 21.11. When all of the wavelengths are combined, as they are in sunlight, visible light appears white. You can learn more about visible light in the chapter "Visible Light" and at the URL below. | text | null |
L_0753 | the electromagnetic spectrum | T_3810 | The only light that people can see is called visible light. It refers to a very narrow range of wavelengths in the electromagnetic spectrum that falls between infrared light and ultraviolet light. Within the visible range, we see light of different wavelengths as different colors of light, from red light, which has the longest wavelength, to violet light, which has the shortest wavelength. You can see the spectrum of colors of visible light in Figure 21.11. When all of the wavelengths are combined, as they are in sunlight, visible light appears white. You can learn more about visible light in the chapter "Visible Light" and at the URL below. | text | null |
L_0753 | the electromagnetic spectrum | T_3811 | Light with the longest wavelengths is called infrared light. The term infrared means "below red." Infrared light is the range of light waves that have longer wavelengths than red light in the visible spectrum. You cant see infrared light waves, but you can feel them as heat on your skin. The sun gives off infrared light as do fires and living things. The picture of a cat that opened this chapter was made with a camera that detects infrared light waves and changes their energy to colored light in the visible range. Night vision goggles, which are used by law enforcement and the military, also detect infrared light waves. The goggles convert the invisible waves to visible images. For a deeper understanding of infrared light, watch the video at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0753 | the electromagnetic spectrum | T_3812 | Light with wavelengths shorter than visible light is called ultraviolet light. The term ultraviolet means "above violet." Ultraviolet light is the range of light waves that have shorter wavelengths than violet light in the visible spectrum. Humans cant see ultraviolet light, but it is very useful nonetheless. It has higher-frequency waves than visible light, so it has more energy. It can be used to kill bacteria in food and to sterilize laboratory equipment (see Figure 21.12). The human skin also makes vitamin D when it is exposed to ultraviolet light. Vitamin D is needed for strong bones and teeth. You can learn more about ultraviolet light and its discovery at this URL: MEDIA Click image to the left or use the URL below. URL: Too much exposure to ultraviolet light can cause sunburn and skin cancer. You can protect your skin from ultraviolet light by wearing clothing that covers your skin and by applying sunscreen to any exposed areas. The SPF, or sun- protection factor, of sunscreen gives a rough idea of how long it protects the skin from sunburn (see Figure 21.13). A sunscreen with a higher SPF protects the skin longer. You should use sunscreen with an SPF of at least 15 even on cloudy days, because ultraviolet light can travel through clouds. Sunscreen should be applied liberally and often. You can learn more about the effects of ultraviolet light on the skin at this URL: MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0753 | the electromagnetic spectrum | T_3813 | The shortest-wavelength, highest-frequency electromagnetic waves are X rays and gamma rays. These rays have so much energy that they can pass through many materials. This makes them potentially very harmful, but it also makes them useful for certain purposes. | text | null |
L_0753 | the electromagnetic spectrum | T_3814 | X rays are high-energy electromagnetic waves. They have enough energy to pass through soft tissues such as skin but not enough to pass through bones and teeth, which are very dense. The bright areas on the X ray film in Figure also to screen luggage at airports (see Figure 21.14). Too much X ray exposure may cause cancer. If youve had dental X rays, you may have noticed that a heavy apron was placed over your body to protect it from stray X rays. The apron is made of lead, which X rays cannot pass through. You can learn about the discovery of X rays as well as other uses of X rays at this URL: | text | null |
L_0753 | the electromagnetic spectrum | T_3815 | Gamma rays are the most energetic of all electromagnetic waves. They can pass through most materials, including bones and teeth. Nonetheless, even these waves are useful. For example, they can be used to treat cancer. A medical device sends gamma rays the site of the cancer, and the rays destroy the cancerous cells. If you want to learn more about gamma rays, watch the video at the URL below. MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0753 | the electromagnetic spectrum | T_3816 | Scientists in Berkeley have developed a powerful new microscope which uses X rays to scan a whole cell and in a manner of minutes, generate a 3D view of the cell and its genetic material. This groundbreaking tool is helping to advance research into the development of biofuels, the treatment of malaria and it may even help to more rapidly diagnose cancer. For more information on X ray microscopes, see http://science.kqed.org/quest/video/x-ray-micros MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0754 | the light we see | T_3817 | Look at the classroom in Figure 22.1. It has several sources of visible light. One source of visible light is the sun. Sunlight enters the classroom through the windows. The sun provides virtually all of the visible light that living things need. Visible light travels across space from the sun to Earth in electromagnetic waves. But how does the sun produce light? Read on to find out. | text | null |
L_0754 | the light we see | T_3818 | The sun and other stars produce light because they are so hot. They glow with light due to their extremely high temperatures. This way of producing light is called incandescence. Some objects produce light without becoming very hot. They generate light through chemical reactions or other processes. Producing light without heat is called luminescence. Objects that produce light by luminescence are said to be luminous. Luminescence, in turn, can occur in different ways: One type of luminescence is called fluorescence. In this process, a substance absorbs shorter-wavelength light, such as ultraviolet light, and then gives off light in the visible range of wavelengths. Certain minerals produce light in this way. Another type of luminescence is called electroluminescence. In this process, a substance gives off light when an electric current runs through it. Some gases produce light in this way. A third type of luminescence is called bioluminescence. This is the production of light by living things as a result of chemical reactions. Examples of bioluminescent organisms are pictured in Figure 22.2. You can learn more about bioluminescence in the video at this URL: Many other objects appear to produce their own light, but they actually just reflect light from another source. The moon is a good example. It appears to glow in the sky from its own light, but in reality it is just reflecting light from the sun. Objects like the moon that are lit up by another source of light are said to be illuminated. Everything you can see that doesnt produce its own light is illuminated. | text | null |
L_0754 | the light we see | T_3819 | The classroom in Figure 22.1 has artificial light sources in addition to natural sunlight. There are fluorescent lights on the ceiling of the room. There are also projectors on the ceiling that are shining light on screens. In these and most other artificial light sources, electricity provides the energy and some type of light bulb converts the electrical energy to visible light. How a light bulb produces visible light varies by type of bulb, as you can see in Table 22.1. Incandescent light bulbs, which produce light by incandescence, give off a lot of heat as well as light, so they waste energy. Other light bulbs produce light by luminescence, so they produce little if any heat. These light bulbs use energy more efficiently. Which types of light bulbs do you use? Type of Light Bulb Incandescent Light Description An incandescent light bulb produces visible light by incandescence. The bulb contains a thin wire filament made of tungsten. When electric current passes through the filament, it gets extremely hot and glows. You can learn more about incandescent light bulbs at the URL below. Fluorescent Light A fluorescent light bulb produces visible light by flu- orescence. The bulb contains mercury gas that gives off ultraviolet light when electricity passes through it. The inside of the bulb is coated with a substance called phosphor. The phosphor absorbs the ultraviolet light and then gives off most of the energy as visible light. You can learn more about fluorescent light bulbs at this URL: http://science.discovery.com/videos/deco Type of Light Bulb Neon Light Vapor Light LED Light Description A neon light produces visible light by electrolumines- cence. The bulb is a glass tube that contains the noble gas neon. When electricity passes through the gas, it excites electrons of neon atoms, causing them to give off visible light. Neon produces red light. Other noble gases are also used in lights, and they produce light of different colors. For example, krypton produces violet light, and argon produces blue light. A vapor light produces visible light by electrolumi- nescence. The bulb contains a small amount of solid sodium or mercury as well as a mixture of neon and argon gases. When an electric current passes through the gases, it causes the solid sodium or mercury to change to a gas and emit visible light. Sodium vapor lights, like these streetlights, produce yellowish light. Mercury vapor lights produce bluish light. Vapor lights are very bright and energy efficient. The bulbs are also long lasting. LED stands for light-emitting diode. This type of light contains a material, called a semi-conductor, which gives off visible light when a current runs through it. LED lights are used for traffic lights and indicator lights on computers, cars, and many other devices. This type of light is very reliable and durable. | text | null |
L_0754 | the light we see | T_3820 | When visible light strikes matter, it interacts with it. How light interacts with matter depends on the type of matter. | text | null |
L_0754 | the light we see | T_3821 | Light may interact with matter in several ways. Light may be reflected by matter. Reflected light bounces back when it strikes matter. Reflection of light is similar to reflection of sound waves. You can read more about reflection of light later on in this chapter in the lesson Optics. Light may be refracted by matter. The light is bent when it passes from one type of matter to another. Refraction of light is similar to refraction of sound waves. You can also read more about refraction of light in the lesson Optics. Light may pass through matter. This is called transmission of light. As light is transmitted, it may be scattered by particles of matter and spread out in all directions. This is called scattering of light. Light may be absorbed by matter. This is called absorption of light. When light is absorbed, it doesnt reflect from or pass through matter. Instead, its energy is transferred to particles of matter, which may increase the temperature of matter. | text | null |
L_0754 | the light we see | T_3822 | Matter can be classified on the basis of how light interacts with it. Matter may be transparent, translucent, or opaque. Each type of matter is illustrated in Figure 22.3. Transparent matter is matter that transmits light without scattering it. Examples of transparent matter include air, pure water, and clear glass. You can see clearly through a transparent object, such as the revolving glass doors in the figure, because all the light passes straight through it. Translucent matter is matter that transmits but scatters light. Light passes through a translucent object but you cannot see clearly through the object because the light is scattered in all directions. The frosted glass doors in the figure are translucent. Opaque matter is matter that does not let any light pass through it. Matter may be opaque because it absorbs light, reflects light, or does both. Examples of opaque objects are solid wooden doors and glass mirrors. A wooden door absorbs most of the light that strikes it and reflects just a few wavelengths of visible light. A mirror, which is a sheet of glass with a shiny metal coating on the back, reflects all the light that strikes it. | text | null |
L_0754 | the light we see | T_3823 | Visible light consists of a range of wavelengths. The wavelength of visible light determines the color that the light appears. As you can see in Figure 22.4, light with the longest wavelength appears red, and light with the shortest wavelength appears violet. In between is a continuum of all the other colors of light. Only a few colors of light are represented in the figure. | text | null |
L_0754 | the light we see | T_3824 | A prism, like the one in Figure 22.5, can be used to separate visible light into its different colors. A prism is a pyramid-shaped object made of transparent matter, usually clear glass. It transmits light but slows it down. When light passes from the air to the glass of the prism, the change in speed causes the light to bend. Different wavelengths of light bend at different angles. This causes the beam of light to separate into light of different wavelengths. What we see is a rainbow of colors. Look back at the rainbow that opened this chapter. Do you see all the different colors of light, from red at the top to violet at the bottom? Individual raindrops act as tiny prisms. They separate sunlight into its different wavelengths and create a rainbow. For an animated version of Figure 22.5, go to the URL: http://en.wikipedia.org/wiki/File:Light_dispersion_conce | text | null |
L_0754 | the light we see | T_3825 | We see an opaque object, such as the apple in Figure 22.6, because it reflects some wavelengths of visible light. The wavelengths that are reflected determine the color that the object appears. For example, the apple in the figure appears red because it reflects red light and absorbs light of other wavelengths. We see a transparent or translucent object, such as the bottle in Figure 22.6, because it transmits light. The wavelength of the transmitted light determines the color that the object appears. For example, the bottle in the figure appears blue because it transmits blue light. The color of light that strikes an object may also affect the color that the object appears. For example, if only blue light strikes a red apple, the blue light is absorbed and no light is reflected. When no light reflects from an object, it looks black. Black isnt a color. It is the absence of light. | text | null |
L_0754 | the light we see | T_3826 | The human eye can distinguish only red, green, and blue light. These three colors of light are called primary colors. All other colors of light can be created by combining the primary colors. As you can see in Figure 22.7, when red and green light combine, they form yellow. When red and blue light combine, they form magenta, a dark pinkish color, and when blue and green light combine, they form cyan, a bluish green color. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram in Figure 22.7. When all three primary colors combine, they form white light. White is the color of the full spectrum of visible light when all of its wavelengths are combined. You can explore the colors of visible light and how they combine with the interactive animations at this URL: . | text | null |
L_0754 | the light we see | T_3826 | The human eye can distinguish only red, green, and blue light. These three colors of light are called primary colors. All other colors of light can be created by combining the primary colors. As you can see in Figure 22.7, when red and green light combine, they form yellow. When red and blue light combine, they form magenta, a dark pinkish color, and when blue and green light combine, they form cyan, a bluish green color. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram in Figure 22.7. When all three primary colors combine, they form white light. White is the color of the full spectrum of visible light when all of its wavelengths are combined. You can explore the colors of visible light and how they combine with the interactive animations at this URL: . | text | null |
L_0754 | the light we see | T_3826 | The human eye can distinguish only red, green, and blue light. These three colors of light are called primary colors. All other colors of light can be created by combining the primary colors. As you can see in Figure 22.7, when red and green light combine, they form yellow. When red and blue light combine, they form magenta, a dark pinkish color, and when blue and green light combine, they form cyan, a bluish green color. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram in Figure 22.7. When all three primary colors combine, they form white light. White is the color of the full spectrum of visible light when all of its wavelengths are combined. You can explore the colors of visible light and how they combine with the interactive animations at this URL: . | text | null |
L_0754 | the light we see | T_3826 | The human eye can distinguish only red, green, and blue light. These three colors of light are called primary colors. All other colors of light can be created by combining the primary colors. As you can see in Figure 22.7, when red and green light combine, they form yellow. When red and blue light combine, they form magenta, a dark pinkish color, and when blue and green light combine, they form cyan, a bluish green color. Yellow, magenta, and cyan are called the secondary colors of light. Look at the center of the diagram in Figure 22.7. When all three primary colors combine, they form white light. White is the color of the full spectrum of visible light when all of its wavelengths are combined. You can explore the colors of visible light and how they combine with the interactive animations at this URL: . | text | null |
L_0754 | the light we see | T_3827 | Many objects have color because they contain pigments. A pigment is a substance that colors materials by reflecting light of certain wavelengths and absorbing light of other wavelengths. A very common pigment is chlorophyll, which is found in plants. This dark green pigment absorbs all but green wavelengths of visible light. It is responsible for capturing the light energy needed for photosynthesis. Pigments are also found in paints, inks, and dyes. Just three pigments, called primary pigments, can be combined to produce all other colors. The primary pigment colors are the same as the secondary colors of light: cyan, magenta, and yellow. The printer ink cartridges in Figure 22.8 come in just these three colors. They are the only colors needed for full-color printing. | text | null |
L_0754 | the light we see | T_3828 | Artist Kate Nichols longed to paint with the iridescent colors of butterfly wings, but no such pigments existed. So she became the first artist-in-residence at Lawrence Berkeley National Laboratory to synthesize nanoparticles and incorporate them into her artwork. From the laboratory to the studio, see how Kate uses the phenomenon known as "structural color" to transform nanotechnology into creativity. For more information on using nanoparticles to create colors, see http://science.kqed.org/quest/video/science-on-the-spot-color-by-nano-the-art-of-kate-nichols/ . MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0755 | optics | T_3829 | Almost all surfaces reflect some of the light that strikes them. The still water of the lake in Figure 22.9 reflects almost all of the light that strikes it. The reflected light forms an image of nearby objects. An image is a copy of an object that is formed by reflected or refracted light. | text | null |
L_0755 | optics | T_3830 | If a surface is extremely smooth, like very still water, then an image formed by reflection is sharp and clear. This is called regular reflection. If the surface is even slightly rough, an image may not form, or if there is an image, it is blurry or fuzzy. This is called diffuse reflection. Both types of reflection are represented in Figure 22.10. You can also see animations of both types of reflection at this URL: http://toolboxes.flexiblelearning.net.au/demosites/serie In Figure 22.10, the waves of light are represented by arrows called rays. Rays that strike the surface are referred to as incident rays, and rays that reflect off the surface are known as reflected rays. In regular reflection, all the rays are reflected in the same direction. This explains why regular reflection forms a clear image. In diffuse reflection, in contrast, the rays are reflected in many different directions. This is why diffuse reflection forms, at best, a blurry image. | text | null |
L_0755 | optics | T_3831 | One thing is true of both regular and diffuse reflection. The angle at which the reflected rays bounce off the surface is equal to the angle at which the incident rays strike the surface. This is the law of reflection, and it applies to the reflection of all light. The law is illustrated in Figure 22.11 and in the animation at this URL: | text | null |
L_0755 | optics | T_3832 | Mirrors are usually made of glass with a shiny metal backing that reflects all the light that strikes it. Mirrors may have flat or curved surfaces. The shape of a mirrors surface determines the type of image the mirror forms. For example, the image may be real or virtual. A real image forms in front of a mirror where reflected light rays actually meet. It is a true image that could be projected on a screen. A virtual image appears to be on the other side of the mirror. Of course, reflected rays dont actually go behind a mirror, so a virtual image doesnt really exist. It just appears to exist to the human eye and brain. | text | null |
L_0755 | optics | T_3833 | Most mirrors are plane mirrors. A plane mirror has a flat reflective surface and forms only virtual images. The image formed by a plane mirror is also life sized. But something is different about the image compared with the real object in front of the mirror. Left and right are reversed. Look at the man shaving in Figure 22.12. He is using his right hand to hold the razor, but his image appears to be holding the razor in the left hand. Almost all plane mirrors reverse left and right in this way. | text | null |
L_0755 | optics | T_3834 | Some mirrors have a curved rather than flat surface. Curved mirrors can be concave or convex. A concave mirror is shaped like the inside of a bowl. This type of mirror forms either real or virtual images, depending on where the object is placed relative to the focal point. The focal point is the point in front of the mirror where the reflected rays intersect. You can see how concave mirrors form images in Figure 22.13 and in the interactive animation at the URL below. The animation allows you to move an object to see how its position affects the image. Concave mirrors are used behind car headlights. They focus the light and make it brighter. They are also used in some telescopes. | text | null |
L_0755 | optics | T_3834 | Some mirrors have a curved rather than flat surface. Curved mirrors can be concave or convex. A concave mirror is shaped like the inside of a bowl. This type of mirror forms either real or virtual images, depending on where the object is placed relative to the focal point. The focal point is the point in front of the mirror where the reflected rays intersect. You can see how concave mirrors form images in Figure 22.13 and in the interactive animation at the URL below. The animation allows you to move an object to see how its position affects the image. Concave mirrors are used behind car headlights. They focus the light and make it brighter. They are also used in some telescopes. | text | null |
L_0755 | optics | T_3835 | The other type of curved mirror, a convex mirror, is shaped like the outside of a bowl. This type of mirror forms only virtual images. The image is always right-side up and smaller than the actual object, which makes the object appear farther away than it really is. You can see how a convex mirror forms an image in Figure 22.14 and in the animation at the URL below. Because of their shape, convex mirrors can gather and reflect light from a wide area. This is why they are used as side mirrors on cars. They give the driver a wider view of the area around the vehicle than a plane mirror would. | text | null |
L_0755 | optics | T_3836 | Although the speed of light is constant in a vacuum, light travels at different speeds in different kinds of matter. For example, light travels more slowly in glass than in air. Therefore, when light passes from air to glass, it slows down. If light strikes a sheet of glass straight on, or perpendicular to the glass, it slows down but passes straight through. However, if light enters the glass at an angle other than 90 , the wave refracts, or bends. This is illustrated in Figure change in speed, the more light bends. | text | null |
L_0755 | optics | T_3836 | Although the speed of light is constant in a vacuum, light travels at different speeds in different kinds of matter. For example, light travels more slowly in glass than in air. Therefore, when light passes from air to glass, it slows down. If light strikes a sheet of glass straight on, or perpendicular to the glass, it slows down but passes straight through. However, if light enters the glass at an angle other than 90 , the wave refracts, or bends. This is illustrated in Figure change in speed, the more light bends. | text | null |
L_0755 | optics | T_3837 | Lenses make use of the refraction of light to create images. A lens is a transparent object, typically made of glass, with one or two curved surfaces. The more curved the surface of a lens is, the more it refracts light. Like mirrors, lenses may be concave or convex. | text | null |
L_0755 | optics | T_3838 | Concave lenses are thicker at the edges than in the middle. They cause rays of light to diverge, or spread apart. Figure 22.16 shows how a concave lens forms an image. The image is always virtual and on the same side of the lens as the object. The image is also right-side up and smaller than the object. Concave lenses are used in cameras. They focus reduced images inside the camera, where they are captured and stored. You can explore the formation of images by a concave lens with the interactive animation at this URL: http://phet.colorado.edu/sims/geometric-opti | text | null |
L_0755 | optics | T_3839 | Convex lenses are thicker in the middle than at the edges. They cause rays of light to converge, or meet, at a point called the focus (F). Convex lenses form either real or virtual images. It depends on how close an object is to the lens relative to the focus. Figure 22.17 shows how a convex lens works. You can also interact with an animated convex lens at the URL below. An example of a convex lens is a hand lens. | text | null |
L_0755 | optics | T_3840 | Mirrors and lenses are used in optical instruments to reflect and refract light. Optical instruments include micro- scopes, telescopes, cameras, and lasers. | text | null |
L_0755 | optics | T_3841 | A light microscope is an instrument that uses lenses to make enlarged images of objects that are too small for the unaided eye to see. A common type of light microscope is a compound microscope, like the one in Figure 22.18. A compound microscope has at least two convex lenses: one or more objective lenses and one or more eyepiece lenses. The objective lenses are close to the object being viewed. They form an enlarged image of the object inside the microscope. The eyepiece lenses are close to the viewers eyes. They form an enlarged image of the first image. The magnifications of all the lenses are multiplied together to yield the overall magnification of the microscope. Some light microscopes can magnify objects more than 1000 times! For more on light microscopes and the images they create, watch the video at this URL: (7:29). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0755 | optics | T_3841 | A light microscope is an instrument that uses lenses to make enlarged images of objects that are too small for the unaided eye to see. A common type of light microscope is a compound microscope, like the one in Figure 22.18. A compound microscope has at least two convex lenses: one or more objective lenses and one or more eyepiece lenses. The objective lenses are close to the object being viewed. They form an enlarged image of the object inside the microscope. The eyepiece lenses are close to the viewers eyes. They form an enlarged image of the first image. The magnifications of all the lenses are multiplied together to yield the overall magnification of the microscope. Some light microscopes can magnify objects more than 1000 times! For more on light microscopes and the images they create, watch the video at this URL: (7:29). MEDIA Click image to the left or use the URL below. URL: | text | null |
L_0755 | optics | T_3842 | Like microscopes, telescopes use convex lenses to make enlarged images. However, telescopes make enlarged images of objectssuch as distant starsthat only appear tiny because they are very far away. There are two basic types of telescopes: reflecting telescopes and refracting telescopes. The two types are compared in Figure 22.19. You can learn more about telescopes and how they evolved in the video at this URL: | text | null |
L_0755 | optics | T_3843 | A camera is an optical instrument that records an image of an object. The image may be recorded on film or it may be detected by an electronic sensor that stores the image digitally. Regardless of how the image is recorded, all cameras form images in the same basic way, as demonstrated in Figure 22.20 and at the URL below. Light passes through the lens at the front of the camera and enters the camera through an opening called the aperture. As light passes through the lens, it forms a reduced real image. The image focuses on film (or a sensor) at the back of the camera. The lens may be moved back and forth to bring the image into focus. The shutter controls the amount of light that strikes the film (or sensor). It stays open longer in dim light to let more light in. For a series of animations showing how a camera works, go to this URL: . | text | null |
L_0755 | optics | T_3843 | A camera is an optical instrument that records an image of an object. The image may be recorded on film or it may be detected by an electronic sensor that stores the image digitally. Regardless of how the image is recorded, all cameras form images in the same basic way, as demonstrated in Figure 22.20 and at the URL below. Light passes through the lens at the front of the camera and enters the camera through an opening called the aperture. As light passes through the lens, it forms a reduced real image. The image focuses on film (or a sensor) at the back of the camera. The lens may be moved back and forth to bring the image into focus. The shutter controls the amount of light that strikes the film (or sensor). It stays open longer in dim light to let more light in. For a series of animations showing how a camera works, go to this URL: . | text | null |
L_0755 | optics | T_3844 | Did you ever see a cat chase after a laser light, like the one in Figure 22.21? A laser is a device that produces a very focused beam of light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up (see Figure 22.21). Laser light is created in a tube like the one shown in Figure 22.22. Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of one wavelength. At each end of the tube is a concave mirror. The photons of light bounce back and forth in the tube off the mirrors. This focuses the light. The mirror at one end of the tube is partly transparent. A constant stream of photons passes through the transparent part, forming the laser beam. You can see an animation showing how a laser works at this URL: (1:12). MEDIA Click image to the left or use the URL below. URL: Besides entertaining a cat, laser light has many other uses. It is used to scan bar codes, for example, and to carry communication signals in optical fibers. Optical fibers are extremely thin glass tubes that are used to guide laser light (see Figure 22.23). Sounds or pictures are encoded in pulses of laser light, which are then sent through an optical fiber. All of the light reflects off the inside of the fiber, so none of it escapes. As a result, the signal remains strong even over long distances. More than one signal can travel through an optic fiber at the same time, as you can see in Figure 22.23. Optical fibers are used to carry telephone, cable TV, and Internet signals. | text | null |
L_0755 | optics | T_3844 | Did you ever see a cat chase after a laser light, like the one in Figure 22.21? A laser is a device that produces a very focused beam of light of just one wavelength and color. Waves of laser light are synchronized so the crests and troughs of the waves line up (see Figure 22.21). Laser light is created in a tube like the one shown in Figure 22.22. Electrons in a material such as a ruby crystal are stimulated to radiate photons of light of one wavelength. At each end of the tube is a concave mirror. The photons of light bounce back and forth in the tube off the mirrors. This focuses the light. The mirror at one end of the tube is partly transparent. A constant stream of photons passes through the transparent part, forming the laser beam. You can see an animation showing how a laser works at this URL: (1:12). MEDIA Click image to the left or use the URL below. URL: Besides entertaining a cat, laser light has many other uses. It is used to scan bar codes, for example, and to carry communication signals in optical fibers. Optical fibers are extremely thin glass tubes that are used to guide laser light (see Figure 22.23). Sounds or pictures are encoded in pulses of laser light, which are then sent through an optical fiber. All of the light reflects off the inside of the fiber, so none of it escapes. As a result, the signal remains strong even over long distances. More than one signal can travel through an optic fiber at the same time, as you can see in Figure 22.23. Optical fibers are used to carry telephone, cable TV, and Internet signals. | text | null |
L_0756 | vision | T_3845 | The structure of the human eye is shown in Figure 22.24. Find each structure in the diagram as you read about it below. The cornea is the transparent outer covering of the eye. It protects the eye and also acts as a convex lens, helping to focus light that enters the eye. The pupil is an opening in the front of the eye. It looks black because it doesnt reflect any light. It allows light to enter the eye. The pupil automatically gets bigger or smaller to let more or less light in as needed. The iris is the colored part of the eye. It controls the size of the pupil. The lens is a convex lens that fine-tunes the focus so an image forms on the back of the eye. Tiny muscles control the shape of the lens to focus images of close or distant objects. The retina is a membrane lining the back of the eye. The retina has nerve cells called rods and cones that change images to electrical signals. Rods are good at sensing dim light but cant distinguish different colors of light. Cones can sense colors but not in dim light. There are three different types of cones. Each type senses one of the three primary colors of light. The optic nerve carries electrical signals from the rods and cones to the brain. | text | null |
L_0756 | vision | T_3846 | As just described, the eyes collect and focus visible light. The lens and other structures of the eye work together to focus a real image on the retina. The image is upside-down and reduced in size, as you can see in Figure 22.25. The image reaches the brain as electrical signals that travel through the optic nerve. The brain interprets the signals as shape, color, and brightness. It also interprets the image as though it were right-side up. The brain does this automatically, so what we see is always right-side up. The brain also tells us what we are seeing. | text | null |
L_0756 | vision | T_3847 | Many people have vision problems. The problems often can be corrected with contact lenses or lenses in eyeglasses. Some vision problems can also be corrected with laser surgery, which reshapes the cornea. Two of the most common vision problems are nearsightedness and farsightedness. You may even have one of these conditions yourself. Both are illustrated in Figure 22.26 and in the video at this URL: (1:08). MEDIA Click image to the left or use the URL below. URL: Nearsightedness, or myopia, is the condition in which nearby objects are seen clearly, but distant objects are blurry. It occurs when the eyeball is longer than normal. This causes images to be focused in front of the retina. Myopia can be corrected with concave lenses. The lenses focus images farther back in the eye, so they are on the retina instead of in front of it. Farsightedness, or hyperopia, is the condition in which distant objects are seen clearly, but nearby objects are blurry. It occurs when the eyeball is shorter than normal. This causes images to be focused in back of the retina. Hyperopia can be corrected with convex lenses. The lenses focus images farther forward in the eye, so they are on the retina instead of behind it. | text | null |
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