Figure 2-42. Image transmission by mirrors.

TM 9-258

Figure 2-43. Image transmission by prism.

c. Comparison. When mirrors are used, the image

(2) The combination of two mirrors at 90 angle

does not invert or revert the image when reflection takes

is reflected through air; when a prism is used, the image

place in the horizontal plane (the figure shows the rear of

is reflected through glass. While being transmitted

the object and the front of the image). If reflection takes

through glass, the paths of the rays of light are not

place in the vertical plane an inverted, reverted image is

deviated because they travel in a medium of constant

obtained.

optical density. These paths are not deviated upon

b. Image Transmission by Prism.

Image

entering or leaving the glass of the prism provided the

transmission by prism can be brought about in practically

light rays strike the surface of the glass in the direction of

the same manner as an image is transmitted by one

the normal, at right angles to the surface.

plane mirror (B, fig 2-36) or two plane mirrors (fig 2-43).

Actually, the reflecting surfaces of a prism are plane

2-22. Focal Length and Focal Plane.

mirrors. These surfaces are silvered when the angle at

a. Focal Point of Convergent Lens. When light

which the light strikes is less than the critical angle (para

strikes and passes through a convergent lens, the rays

2-16) of the material from which the prism is made; the

from a point of light intersect at a point on the opposite

surfaces require no reflecting agent when this angle is

side of the lens. The point at which the rays intersect is

greater than the critical angle.

termed the focal point.

b. Focal Point-Location. The focal point of a lens

will vary in relation to the distance of the

2-30