Figure 2-21. Projection by parabolic mirror.

TM 9-258

d. With a parabolic mirror, light rays emanating

from an infinitely small source located at the focal point

would be parallel after reflection, since this is the

principal focal point of the parabolic mirror (fig 2-21). In

practice, however, the source of light, which may be a

filament or an arc, is located in the principal point of

focus and the rays diverge as there cannot be a true

point source. All rays falling upon the parabolic mirror,

except those which are diffused or scattered, are

reflected nearly parallel to each other. This allows a long

powerful beam of light to be formed which diverges very

slightly.

Figure 2-21. Projection by parabolic mirror.

Section III. REFRACTION

surfaces of this plate are parallel and air contacts both

2-10. Refraction Through a Glass Plate.

surfaces. The glass and the air are transparent but the

a. What happens when light is refracted can be

glass is optically denser than the air. Light travels

understood by imagining a beam of light, indicated by a

approximately one-third slower in glass than in air.

broad "light ray" or "light beam" (fig 2-22), passing in

slow motion" through a sheet of glass. Both plane

Figure 2-22. Path of beam of light through sheet of glass.

2-15