TC 9-64 _________________________________________________________________________
1-107. Without refraction, light waves would pass in straight lines through
transparent substances without any change of direction. Refer back to figure
1-10, which shows refraction of a wave. As you can see, all rays striking the
glass at any angle other than perpendicular are refracted. However, the
perpendicular ray, which enters the glass normal to the surface, continues
through the glass and into the air in a straight line--no refraction takes
place.
Diffusion of Light
1-108. When light is reflected from a mirror, the angle of reflection of each
ray equals the angle of incidence. When light is reflected from a piece of plain
white paper, however, the reflected beam is scattered, or diffused, as shown
in figure 1-21. Because the surface of the paper is not smooth, the reflected
light is broken up into many light beams that are reflected in all directions.
Figure 1-21. Diffusion of Light
Absorption of Light
1-109. As illustrated in figure 1-21, a light beam is reflected and diffused
when it falls onto a piece of white paper. If a light beam falls onto a piece of
black paper, the black paper absorbs most of the light rays and very little
light is reflected from the paper. If the surface on which the light beam falls
is perfectly black, there is no reflection; that is, the light is totally absorbed.
No matter what kind of surface light falls on, however, some of the light is
absorbed.
COMPARISON OF LIGHT WAVES WITH SOUND WAVES
1-110. There are two main differences between sound waves and light waves.
The first difference is in velocity. Sound waves travel through air at the speed
of approximately 1,100 feet per second; light waves travel through air and
empty space at a speed of approximately 186,000 miles per second (300,000,000
meters per second). The second difference is that sound is composed of
longitudinal waves (alternate compressions and expansions of matter) and
light is composed of transverse waves in an electromagnetic field.
1-111. Although both are forms of wave motion, sound requires a solid,
liquid, or gaseous medium, whereas light can travel through empty space.
The denser the medium, the greater the speed of sound. The opposite is true
of light. Light travels approximately one-third slower in water than in air.
Sound travels through all substances, but light cannot pass through opaque
materials.
1-30
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