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Chapter 3:
Light Waves and How They Behave
Physical Optics In the previous chapter we considered the properties of a light beam regarded as a group of rays. The paths of these rays through a lens produce its well-known focal properties and aberrations. However, light is actually a form of wave motion, and the study of the wave nature of light constitutes the science of physical optics. Some of the wave properties of light are of particular interest to the photographer, and these will be outlined in the present chapter. Velocity, Frequency, and Wavelength Any moving wave train must possess a velocity, a frequency, and a wavelength. In the case of light, these quantities are very extreme compared with their values for some of the other forms of wave motion with which we are familiar, such as sound or water waves. The velocity is extremely high, namely, 186,000 miles per second in vacuum and about two-thirds of that velocity in a transparent substance such as glass. The frequency of light waves, or the number of waves passing a given point in a second, is also extremely high, being of the order of five hundred million million vibrations a second. The exact figure varies somewhat with the color of the light, from about 4 x— 1014 vibrations per second for the red end of the spectrum to over 7 x— 1014 vibrations per second in the violet region. The wavelength of light, or the distance from crest to crest in the wave train, also varies with color, its value in vacuum being about 0.75 micrometers in the deep red and 0.40 micrometers in the extreme violet. (A micrometer, abbreviated μm, is one-thousandth of a millimeter, or one twenty-five thousandth of an inch.)
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