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5

Prisms

When the rays of light pass through a piece of material as shown in
Figure 5.1, travelling from air, it bends towards the normal. Then it leaves
the material travelling back into the air and bends away from the normal.
In this figure the sides of the material are parallel to each other, so the rays
emerges travelling in the same direction as its original travel – it has been
displaced, but not deviated.

Fig. 5.1: The ray appears to be displaced

In the Figure 5.2, the sides of the material are not parallel to each other,
so the ray emerges travelling in a different direction than its original travel.
This happens to every ray that passes through a refracting medium bordered
by two plane surfaces which are inclined at a finite angle. The medium is
called Prism and the angle between the two surfaces is called the apical
angle or refracting angle. A line bisecting the angle is called the axis of the
prism and the opposite surface is called the base of the prism.
The prism, therefore, can be defined as the wedge-shaped lens material
which deviates light towards its base without changing its vergence. All
varieties of spectacle lens has the effect of a prism when viewed through a
point away from the optical center. The further the away from the optical
center, the greater is the prismatic effect.