Prisms - Geometrical and Visual Optics - page 150

I

α

d

O

Figure 10-1. A light ray originating from an object, O, is refracted by a prism so that
it appears to come from a virtual image, I. The angle that the ray emerging from the
prism is deviated from its original course is the angle of deviation, d . The prism's apical
angle is labeled α .

Figure 10-2 also shows the behavior of a thin prism. Although the angle of devia-
tion for the thin prism is dependent on the angle of incidence, it is less so than for
the thick prism, and there is a fairly broad range of incident angles over which it
remains relatively stable. That is, the extensive trough for the thin prism shows that
the minimum angle of deviation holds true for a wide range of incident angles. For
a thin prism in air, the relationship between the minimum angle of deviation, the
apical angle, and index of refraction is as follows:

d min = α ( n − 1)

Thick prism

Thin prism

Angle of incidence

Figure 10-2. This schematic representation shows the angle of deviation for a thick
and thin prism as a function of the angle of incidence. As discussed in the text, the
angle of incidence is less important for thin prisms.