Consider the following statements for refraction of light through a prism, when the angle of deviation is minimum.
(A) The refracted ray inside the prism becomes parallel to the base.
(B) Larger angle prisms provide smaller angle of minimum deviation.
(C) Angle of incidence and angle of emergence become equal.
(D) There are always two sets of angle of incidence for which deviation will be the same, except at the minimum deviation setting.
(E) Angle of refraction becomes double of the prism angle.
Choose the correct answer from the options given below:
Answer: (A) A, C and D only
At minimum deviation the ray passes symmetrically:
- (C) $i = e$ ✔
- (A) $r_1 = r_2 = A/2$, so for an isosceles prism the ray inside is parallel to the base ✔
- (D) Because the path is reversible, angles $i$ and $e$ can be swapped to give the same deviation; the two coincide only at minimum deviation ✔
- (E) The angle of refraction is $r = A/2$, half the prism angle, not double ✘
- (B) $\delta_m = 2\sin^{-1}\!\left(\mu\sin\frac A2\right) - A$ increases with $A$, so larger prisms give larger minimum deviation ✘
Answer: A, C and D only.
Solution by Sreeraj P, M.Sc Physics