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Prisms 43

CHARACTERISTICS OF PRISM
• A prism has a thickest edge, the base and a thinnest edge, the apex.
• A prism displaces the incident rays towards the base of the prism.
• A prism displaces the image towards the apex of the prism.
• A prism does not change the vergence of the rays.
• A prism does not magnify or minify the image.
• A prism also disperses incident pencil rays into its component colours.

DEVIATION PRODUCED BY PRISM
The deviation produced by a prism is measured in terms of prism dioptre
and is given by the symbol Δ. A prism with a power of 1 prism dioptre
deviates light by 1 cm measured at a distance of 1 metre from the prism.
Similarly, a prism of 2 prism dioptre produces a deviation of 2 cm measured
at a distance of 1 metre from the prism. The deviation so produced can be
measured by the Prentice’s Rule:

Deviation in cms
Prism diopter = Distance in metres

E.g: If the light is deviated 6 metres at a distance of 3 metres, then the prism
diopter would be:
0.6
P= cms
3
or, P = 0.2Δ

PRISM ORIENTATION
Prism can be oriented in front of the eye using notations base-in or base-
out, base up or base down when the directions are horizontal and vertical.
All other base directions require 360° notation. E.g: RE 2Δ at 160°.

Fig. 5.4: Orientations of prism