Consider the prism illustrated in the figure 12-2. The angle Aof a prism is known
as the apex angle or refracting angle. The angles associated with a ray of light
entering or leaving the sides of a prism are measured with respect to a normal line
constructed to a side of the prism and the ray of light is governed by Snell’s law^1
entering
nasin x=ngsin αleaving
ngsin β=nasin ξ(12 .11)where nmdenotes the refractive index for light in medium m(m=aair and m=g
glass). Here x is called the angle of incidence and αis called the angle of refrac-
tion. The refracted ray travels across the prism and again undergoes Snell’s law
and becomes the exiting ray. There is a reversibility principle whereby light can
travel in either direction along the path illustrated in the figure 12-2. In figure 12-2
the incident ray is extended and the exiting ray also has been extended and these
extended rays intersect in the angle ycalled the angle of deviation.
Figure 12-2. Ray of light through a prism.
(^1) See Chapter 2 , pages 122-124.