Figure 2-35. Angles of light rays from an underwater source.

TM 9-258

e. Consider a prism with two faces at right angles

c. All rays traveling in an optically dense medium

and striking the surface with an angle of incidence that is

and a back or hypotenuse at a 450 angle (A, fig 2-36).

less than the critical angle of the mediums are refracted

An incident ray, striking one of the right-angle faces in

and pass into the optically lighter medium in accordance

the direction of the perpendicular or the normal, passes

with the laws of refraction. All such rays striking at an

into the prism without refraction or deviation until it hits

angle of incidence greater than the critical angle of

the boundary of the medium (back of the prism). Since

mediums are reflected inwards in accordance with the

the angle of the back is 450 and the critical angle of the

glass of the prism is 420, the ray cannot pass through

d. The critical angle has a practical application in

the back surface of the prism. Instead, it is totally

the design of prisms used in fire-control instruments. In

reflected. Inasmuch as the angle of incidence between

such instruments, it is quite often necessary to change

the incident ray and the normal (at the back surface) is

the course of the path of light. This deviation could be

450, the angle of reflection would be the same because

accomplished by mirrors but the prisms used perform the

these angles are always equal. The result is that the ray

task more satisfactorily. When prisms are employed at

is reflected a total of 900, strikes the surface of the other

angles greater than the critical angle of the substance of

right-angle face of the prism in the direction of the

which they are made, their reflecting surfaces do not

normal, and passes out of the prism without further

require silvering, yet these surfaces appear to be silvered

deviation.

when one looks into such a prism.

2-24