Section VI. CHARACTERISTICS OF OPTICAL SYSTEMS

TM 9-258

reflection.

2-28. Selective Reflection and Absorption.

b. An object appears to be of a color because it

a. Objects derive their color from the selective

absorbs light of certain wavelengths and reflects others

reflection and selective absorption of light. If an object

striking it. In sunlight, it may appear to be of one color

reflects practically all of the light striking it, the object

because it reflects the light of that color and absorbs all

appears white. If it absorbs the greatest percentage of

others. Under a single color or monochromatic light, it

light, it appears black. The surfaces of most objects

might appear to be of another color or almost black,

absorb light of certain frequencies and reflect light of

depending upon what colors and how much light it

other frequencies. The colors of these surfaces are

reflected.

determined by the frequencies they reflect. This is

known as selective

Section VI. CHARACTERISTICS OF OPTICAL SYSTEMS

eyepiece magnifies the image formed by the objective, it

2-29. General.

Military fire-control instruments

cannot supply sharpness or detail lacking in the image.

employing optical systems are required for a great

The shorter the focus of the eyepiece, the greater is the

variety of purposes. To cite a few, these instruments are

magnifying power.

used to see at great distances, to magnify small objects,

to determine positions, to gage distance, and to aim

2-32. Field of View.

weapons.

a. The true field of view of any optical instrument is

the extent (the width and height) of what can be seen at

2-30. Military Telescopes. Necessary characteristics

one time by looking through the instrument. The field of

of military optical instruments are usually the following a

view is cone-shaped; it becomes wider and higher as the

minimum of weight and bulk, the largest possible field of

distance is increased from the objective. In the majority

view and brightness of image consistent with the

of fire-control instruments the angle of the true field of

necessary magnifying power; freedom from distortion;

view is relatively small (para 5-25).

and a combination of ruggedness with simplicity. The

b. The true field is limited by the optical possibilities

instrument must form a normal erect image of the object.

of the eyepiece(fig. 2-52). The maximum angle which

In the majority of instruments, the optical system must

might be imaged by the eyepiece, if it were not for a

include a reticle pattern.

number of limiting factors, is termed the apparent field of

view (para 5-26)

2-31. Magnification. Magnification of a telescope

optical system depends both upon the objective and the

eyelens or eyepiece. Although the

Figure 2-52. Field of view limited by optical possibilities of eyepiece.

2-39