Figure 2-19. Reflection from concave mirror.

TM 9-258

b. It will be noted that the rays originating from an

infinite (distance) source, such as the sun, are

converging after reflection and that they intersect very

close to a point halfway between the center of curvature

and the surface of the mirror. This point is known as the

principal focal point of the mirror. This point of principal

focus is always one-half of the distance from the center

of curvature to the surface of a concave mirror, provided

the mirror is a true portion of a sphere. Due to "spherical

aberration, " the principal focus is a true point only for a

small central bundle of rays. After passing through the

focal point, the light will diverge.

c. If a very small luminous source is located at the

principal point of focus, the rays will be nearly parallel

after reflection. This is true only if the curvature is very

slight. Actually, the rays will have a slight convergence,

especially those which are reflected from near the edges

Figure 2-20. Projection by spherical mirror.

of the mirror (fig 2-20). For this reason, if it is desired to

have practically parallel rays after reflection, a parabolic

mirror is used.

2-14