Figure 2-24. Terms used with reference to refracted light.

TM 9-258

b. The greater the angle at which the light strikes

original path by the optical density of the refracting

medium. The angle formed by the refracted ray with the

the medium and the greater the difference in optical

normal is called the angle of refraction. The incident ray,

density between the two mediums, the greater the

refracted ray, emergent ray, and the normal all lie in the

bending. If the faces of the medium are parallel, the

bending at the two faces is always the same so that the

d. The angle of refraction depends on the relative

beam which leaves the optically denser medium is

optical density of the two mediums as well as on the

parallel to the incident beam (A, fig 2-23).

c. The ray which strikes the surface is called the

angle of incidence.

e. An equation known as Snell's law may be used

incident ray (fig 2-24). The ray entering the second

to determine the angle of refraction (r) if the angle of

medium is the refracted ray. The ray leaving the second

incidence (i) and the index of refraction (para 2-12c) of

medium is the emergent ray. An imaginary line at right

each medium are known (fig 2-25).

angles to the surface of the medium at the point where

sin i

the ray strikes or leaves the surface is termed the normal

This equation is

= n where n is the sin r

or perpendicular. The angle between the incident ray

sin r

and the normal is the angle of incidence. The angle

quotient of the indices of refraction of the two mediums.

between the refracted ray and an extension of the

If one of the mediums is air, n is practically the index of

incident ray is termed the angle of deviation. It is the

refraction of the second medium.

angle through which the refracted ray is bent from its

2-17