Electromagnetic Spectrum
Our eyes are sensitive to only a very limited portion of the entire range of wavelengths for electromagnetic radiation. If we order this range of wavelengths from the shortest on the left to the longest on the right, we produce an array of wavelengths called the electromagnetic spectrum, which is shown in Figure 4.4. Toward the short-wavelength end we find that portion to which our eyes are sensitive, called the visible spectrum. The physiological response of the eye to the various wavelengths of the visible spectrum is color.
Short wavelengths in the visible spectrum are violet, with progressively longer wavelengths producing the response we identify as the range of hues from blue, green, yellow, and orange to red in the color spectrum. Visible light is electromagnetic radiation with wavelengths between approximately 35 x 10-6 and 70 x 10-6 centimeter. These wavelengths correspond to frequencies between 8.5 x 10'4 to 4.3 X 10'4 hertz. One hertz equals one cycle, or oscillation, of the wave per second. The lowest frequencies of visible light appear red to our eyes; the highest frequencies appear violet; and between these are the rest of the color spectrum.
All types of electromagnetic radiation show the properties of a wave; all propagate in the same way with the same speed in empty space; and all transport energy. For convenience, however, we divide the nonvisible portions of the electromagnetic spectrum into regions according to the radiation's wavelength, such as the ultraviolet or the infrared and so on. We label these different regions not because of any intrinsic difference in the radiation but because we have different ways of detecting radiation depending on its wavelength.
Gamma rays, X rays, and ultraviolet radiation are the wavelength regions shorter than visible light; most of this radiation that comes from outer space is absorbed high above the earth's surface by our atmosphere. Infrared radiation is the first wavelength region beyond visible light, and it is also partially absorbed by the earth's atmosphere. The next wavelength regions are the microwave and radio regions, for which there is a broad electromagnetic window (Figure 5.1) through the earth's atmosphere.
Because of the wide range of wavelengths, some units of measurement are more convenient than others for describing different regions of the electromagnetic spectrum. For the visible spectrum angstroms are convenient. An angstrom is a hundred-millionth of a centimeter (1 A = 10-8 cm). Visible radiation lies approximately between 3500 angstroms (the violet end of the spectrum) and 7000 angstroms (the red end). X rays are also measured in angstroms, but infrared wavelengths are generally expressed in microns (1 micron = 104 angstroms = 10-4 centimeter). Astronomers use the hertz as the unit for measuring frequency of all radiation.
