rowave oven. A second com- mon use for microwaves occurs at frequencies around 10 GHz. Microwaves at that frequency are used for radar, where reflected microwaves directed at a specific target are returned at a shifted fre- quency. That returned frequency, which results from the Doppler effect, enables the speed of the object to be determined, to the chagrin of many motorists. INFRARED Infrared waves occur in that portion of the frequency spectrum above the microwave region but below visible light. Infrared rays are generated by hot objects in the form of infrared radiation, which explains how devices with sensors tuned to the infrared region can detect cars, tanks, and other vehicles and pinpoint the loss of heat from homes and apartments. Another use for infrared is for short-dis- tance communications; for instance, many laptop and notebook comput- ers have an infrared sensor. VISIBLE LIGHT As mentioned earlier, visible light resides in the elec- tromagnetic spectrum with wavelengths between 400 and 700 billionths of a meter (400 to 700 nm). To better appreciate its positioning in the fre- quency spectrum, refer to Figure 2.4, which provides an "exploded" view Chapter Two of the frequency spectrum indicating the approximate wavelengths for red and violet light, which represent the boundaries of visible light. Note that red light has a wavelength of 680 nm while violet light has a wavelength of 410 nm. Because every electromagnetic wave has a unique frequency and wave- length associated with it, we can draw the wave corresponding to each color of light. For example, Figure 2.5 illustrates the electromagnetic wave corresponding to the color red. Note that its frequency is 428.570 GHz, or 428,570 billion cycles per second. Thus, when you view red light, your eye receives over 400 trillion waves per second. Also note that the wavelength of red light is 700 nm long, which is 7 ten-millionths of a meter. When we discuss the operation of lasers later in this book, we'll see that the wavelength employed varies with the