Lesson 8 of 11 · 11 min
Refraction through a prism
NCERT § "Refraction through a Prism"
Kabir sends a narrow beam into a glass prism with 60° faces and turns it slowly. The spot on the wall moves in, stops, then moves back out, even though he keeps turning the same way.
The lesson in notes
In short
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.