Figure 2-6. Waves and radii from nearby light source.

TM 9-258

Figure 2-7. Waves and radii from distant light source.

(5) Principle of reversibility of light paths. If a

single narrow beam of parallel light is reflected or

refracted in any combination through a number of optical

elements, it will retrace its path through the elements if

the light were to enter the optical system from the other

end. This is true no matter how many reflections and

refractions the light has undergone.

(6) Color.

Most substances reflect light

selectively. Certain wavelengths (colors) are reflected,

while others are absorbed. The color of an object is the

one which is reflected. A colored transparent medium

(filter) transmits light selectively, absorbing all rays not

transmitted.

(7) Biological effects. Infrared light is not very

effective on living tissues except to produce heat. Visible

light is beneficial in its effects. Shorter waves may

produce serious burns, but possess marked bactericidal

properties and are used for sterilizing milk and other

materials. In general, the shorter the waves the more

violent are they in their effects on living tissues.

Figure 2-6. Waves and radii from nearby light source.

2-2. Speed of Light

a. The speed of light (fig 2-8) is an important factor

in the study of the physical nature of light., The lengths of

all waves in the electromagnetic spectrum (fig 2-3) are

logically connected to corresponding frequencies and to

the speed of light (para 2-3). The difference in the speed

of light through air, glass, and other substances accounts

for the bending of light rays or refraction.

2-6