Figure 2-11. Symbols and types of illustrations used in this manual.

TM 9-258

b. Single rays of light do not exist. The term "light

from a luminous or illuminated point, these lightrays

indicate a solid cone of light from that point (A, fig 2-11).

ray" is used throughout this manual for the sake of clarity

In certain of the illustrations, an attempt has been made

and convenience to indicate the direction of travel of

to simulate height, width, and depth by preparing the

light.

c. In studying the diagrams, it must be

diagrams to appear as though viewed from an angle (A,

remembered that printed illustrations have only height

d. The letter F is used as the object in a great many

and width (b, fig 2-11) while everything, including light,

of the diagrams because it shows quickly whether the

has three dimensions: height, width, and depth (or

image of this letter or object is upright, inverted, or

length). The reader's imagination must be employed to

reverted.

give the illustration depth. Thus, for example, where two

light rays are used to indicate the course of light

Section II. REFLECTION

directed back along the path of the incoming beam (A,

2-5.

Reflection From a Plane Mirror.

fig 2-12). If the mirror is shifted to an angle from this

a. A plane mirror is a flat polished surface used to

position, the reflected beam will be shifted at an angle

reflect light. If a beam of light is permitted to enter a

from the incoming beam that is twice as great as the

darkened room and a plane mirror is held so that the

angle by which the mirror is shifted (B, fig 2-12). For

beam will strike the mirror, the beam will be reflected. By

example, if the mirror is held at angle of 45 to the

shifting the mirror, the beam can be reflected to almost

incoming beam, the reflected beam will be projected at

any part of the room.

an angle of 90 to the incoming beam.

b. If the mirror is shifted so that it is exactly at right

angles (normal) to the beam, the reflected beam will be

2-9