Simulation · Physics · Class 12

Prism: the angle of minimum deviation

From the lesson Refraction through a prism in Ray Optics and Optical Instruments. Change the values and watch what happens.

The idea behind it

NCERT § "Refraction through a Prism"

For a prism of refracting angle A, the angles of refraction at the two faces satisfy r₁ + r₂ = A, and the deviation is δ = i + e − A.
As the angle of incidence increases, the deviation first falls, reaches a minimum value D_m, and then rises again.
At minimum deviation the ray passes symmetrically through the prism: i = e and r₁ = r₂ = A/2; for the usual isosceles or equilateral prism the ray inside then runs parallel to the base.
Refractive index from minimum deviation: n₂₁ = sin[(A + D_m)/2] / sin(A/2).
For a thin prism (small A), D_m = (n₂₁ − 1)A, so the deviation does not depend on the angle of incidence for small angles.
Deviation depends on the refractive index of the prism relative to the surrounding medium; the same prism deviates light less when immersed in a liquid whose refractive index is lower than the prism's.

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