Figure 2-28. Path of light ray through prism.

TM 9-258

d. Over large areas of heated sand or over water,

of glass with parallel surfaces because the light which

penetrates the base or thicker part of the prism must

conditions are such as to produce strata or layers of air

travel longer and at less speed in an optically denser

differing greatly in temperature and refractive index.

medium than the light passing through the apex or

Under such conditions, erect or inverted and sometimes

thinner part of the prism. Rays of light emerging from a

much distorted images are formed which can be seen

plate of glass with parallel surfaces always travel in a

from a great distance. These images are known as

parallel direction with the incident rays; rays emerging

mirages.

e. On a hot day, the columns of heated air rising

from a prism always travel at different angles from the

incident rays.

from the earth are optically different from the surrounding

b. The laws of refraction apply to prisms just as

air and rays of light are irregularly refracted. The air is

they do to plates of glass with parallel surfaces. These

turbulent and conditions under which observations are

laws may be applied to plot the paths of light through any

made are changing all the time. Consequently, an object

prism.

viewed through such layers of air appears to be in motion

c. When a ray of light strikes the surface of a

about a mean position. In such cases the air is said to

prism, the refracted ray (fig 2-28) is bent toward the

be "boiling" or the image is "dancing" due to "heat

normal according to the law of refraction. The refracted

waves." This condition is particularly detrimental when a

ray is bent away from the normal on leaving the prism.

high-power telescope is employed.

Under such

Thus, the path of the ray is deviated at the first surface of

conditions it is usually impossible to use an instrument of

the prism and then further deviated at the second

more than 20 power.

surface. In both cases, the ray is bent toward the

2-14. Refraction Through Triangular Glass Prism.

thickest part of the prism.

a. Unlike a plate of glass, a prism has its faces cut

at an angle. Refraction through a triangular glass prism

differs

from

refraction

through

sheet

Figure 2-28. Path of light ray through prism.

front surfaces will pass through the prisms. The rays

2-15.

Refraction Through Lenses.

emerging from one prism will cross the emergent rays of

Convergent Lens. If two prisms are

the other prism.

arranged base to base (fig 2-29), rays of light striking the

2-20