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42 Ophthalmic Lenses

Fig. 5.2: Rays emerging, travelling in different direction

Look at the Figure 5.3, one plane on the edge, i.e. the base of the prism
is thicker than any other portion and the plane opposite to base is the
thinnest – known as apex of the prism. The thickest edge is exactly opposite
the thinnest edge (provided prism has no power).

Fig. 5.3
When the prisms are prescribed, the orientation of it, is indicated by
the position of base, i.e., ‘base - in’, ‘base - out’ and so on. Light rays entering
and leaving a prism are bent towards the base of the prism. This causes
objects to be displaced away from the base of the prism towards its apex.
Thus, if an object is viewed through a base down prism, it will be seen to be
displaced upward. Similarly, objects appear to shift downward when
viewed through a base up prism, to the right or left when seen through a
base-out or base-in prism.
The amount by which a ray will be deviated when it passes through the
prism depends on:
• The apical angle
• The index of refraction of the material
• The wavelength of the ray
• The angle from which the ray approaches the prism.