Section IV. IMAGE FORMATION

TM 9-258

b. The small critical angle of diamond accounts for

substance. The light entering the diamond is totally

reflected back and forth a great number of times before

the brilliance of a well-cut diamond. It is due to a total

emerging, producing bright multiple reflections.

internal reflection of light which occurs for a greater

variety of angles than in any other

Section IV. IMAGE FORMATION

(2) Real images. A real image actually exists.

2-18. General. In considering the principles of the

It can be thrown upon a screen. The lenses of the

formation of images by lenses, for simplicity of

human eye and the camera form real images. The real

explanation it will be assumed that perfect single lenses

image formed by a camera can be seen on the ground

are used.

2-19. Classification of Images.

a. Virtual and Real Images. Two types of images

are produced by optical elements, virtual images and real

images.

(1) Virtual Images. A virtual image is so called

because it has no real existence. It exists only in the

mind and cannot be thrown upon a screen because it is

apparent only to the eyes of the observer. A familiar

example is the virtual image formed by a mirror. The

image of the person looking into the mirror appears to be

on the other side of the mirror (fig 2-37). No trace of the

image can be seen on the back of the mirror. A plane

mirror produces a virtual image resulting in an optical

illusion in which the object appears to be behind the

mirror.

Figure 2-38. Real image formed on ground glass of

camera.

b. Erect, Inverted, Normal, and Reverted Images.

When an image is reflected or refracted by optical

elements, the parts of the object may be seen as being

transposed horizontally, vertically, or both. This is

illustrated by what happens to the letter F shown in figure

Figure 2-37. Virtual image reflected by plane mirror.

2-26