________________________________________________________________Wave Propagation
Figure 1-17. Waves and Radii from a Nearby Light Source
1-88. A large volume of light is called a beam; a narrow beam is called a
pencil; and the smallest portion of a pencil is called a light ray. A ray of light
can be illustrated as a straight line. This straight line drawn from a light
source represents an infinite number of rays radiating in all directions from
the source.
FREQUENCIES AND WAVELENGTHS
1-89. Compared to sound waves, the frequency of light waves is very high
and the wavelength is very short. To measure these wavelengths
conveniently, a special unit of measure called an angstrom unit, or more
usually, an angstrom (), was devised. Another common unit used to
measure these waves is the millimicron (mm), which is one millionth of a
millimeter. One mF equals ten angstroms. One angstrom equals 10-10 meters.
FREQUENCIES AND COLOR
1-90. For our discussion of light wave waves, we use the millimicron
measurement. The wavelength of a light determines the color of the light.
Figure 1-18 indicates that light with a wavelength of 700 millimicrons is red,
and that light with a wavelength of 500 millimicrons is blue-green. This
illustration shows approximate wavelengths of the different colors in the
visible spectrum. In fact, the color of light depends on its frequency, not its
wavelength. However, light is measured in wavelengths. When the
wavelength of 700 millimicrons is measured in a medium such as air, it
produces the color red, but the same wave measured in a different medium
will have a different wavelength. When red light that has been traveling in
air enters glass, it loses speed. Its wavelength becomes shorter or
compressed, but it continues to be red. This phenomenon indicates that the
color of light depends on frequency and not on wavelength. The color scale in
figure 1-18 is based on the wavelengths in air.
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