Figure 2-25. Index of Refraction.

TM 9-258

determined from the equation of Snell's law (para 2-11e)

by actually measuring the angles i and r in a simple

experiment, inasmuch as air can be used instead of

vacuum for all practical purposes.

c. The following listing contains the indices of

refraction of a number of substances.

NOTE

For most computations, the index of air

is considered to be unity (1.000). The

error introduced equals approximately

0.03 percent.

Vacuum .................................................... 1.000000

Air ............................................................. 1.000292

Water .............................................................. 1.333

Boro-silicate crown glass ................................ 1.517

Thermosetting cement .................................... 1.529

Thermoplastic cement

(Canada balsam) ..................................... 1.530

Gelatin ............................................................ 1.530

Light flint glass ................................................ 1.588

Medium flint glass ........................................... 1.617

Figure 2-25. Index of Refraction.

Dense flint glass ............................................. 1.649

Densest flint glass .......................................... 1.963

2-12. Index of Refraction.

2-13. Atmospheric Refraction

a. Light travels through substances of different

a. At a surface separating two media of different

optical densities with greatly varying velocities. For

indices of refraction, the direction of the path of light

example, the speed of light in air is approximately 186,

changes abruptly when passing through the surface. If

000 miles per second; in ordinary glass, it is

the index of refraction of a single medium changes

approximately 120, 000 miles per second.

gradually as the light proceeds from point to point, the

b. The ratio between the speed of light in vacuum

path of the light will also change gradually and will be

and the speed of light in a medium is known as the index

curved rather than in a straight line.

of refraction. The index of refraction is usually indicated

b. Although air at its densest has a refractive index

by letter n and is determined by dividing the speed of

on only 1.000292, this is sufficient to bend light rays from

light in a vacuum by the speed of light in the particular

the sun toward the earth when these rays strike the

medium.

atmosphere at an angle (fig 2-26).

The earth's

Velocity in Vacuum

atmosphere is a medium which becomes denser toward

Index of Refraction =

the surface of the earth. The result is that of light

Velocity in Medium

traveling through the atmosphere toward the earth at an

The index of refraction n, of a substance, can be

angle does not travel in a straight line but is refracted

and follows a curved path. From points near the horizon,

the bending of light is so great that the setting sun in

seen after it is completely below the horizon (fig 2-27).

2-18