WAVELENGTH nown frequency (HERTZ) COSMIC RAYS (METERS) GAMMA X-UNIT, XU PICOMETER 10-11 X-RAYS 10-10 ANGSTROM, a NANO- HERTZ METER, nm ULTRAVIOLET PENTA- 1015 MICRO- HERTZ VISIBLE LIGHT METER, um Fiber Optic INFRARED Communications TERA- MILLI- HERTZ METER, mm CENTI- MICROWAVES METER, cm GIGA- METER, m HERTZ UHF TV VHF TV Mobile Radio MEGA- 106 Shortwave Radio KILO- HERTZ METER, km KILO- HERTZ AUDIO HERTZ 1 Chapter Two or a billionth, of meters (10-9 m). Thus, red light is normally referred to as having a wavelength of 680 nm. In examining the frequency spectrum shown in Figure 2.3, note that the rainbow of colors we're able to see represents only a very small por- tion of the electromagnetic spectrum. At the low end of the spectrum are radio waves that have wavelengths billions of times longer than those of visible light. In fact, visible-light wavelengths are considerably smaller than the thickness of a human hair, whose diameters might be equiva- lent to 100 visible-light wavelengths. While visible light has a relatively small wavelength, X rays and gamma rays are even shorter in length and, similar to longer wavelengths, are invisible to the naked eye. In fact, radi- ation with wavelengths shorter than 400 nm or longer than 700 nm are invisible to the naked eye. If we look very closely at the right side of Figure 2.3, we will note that the wavelength from approximately 10-3 to 10-8 m is referred to as the electrooptical frequency (EOF) band. Within this band are infrared and visible light. We will also note a tiny horizontal line below the visible- light band labeled fiber-optic communications. When we discuss the use of light-emitting diodes (LEDs) and laser diodes later in this book, we will note that the wavelengths commonly used by those light generators are slightly above the wavelength of visible light. In Figure 2.3 we will also note that visible light represents only a small portion of the electromagnetic spectrum. That spectrum consists of seven types of waves: radio waves, microwave, infrared, visib