Which of the following statements are true: For refraction of white light through a glass prism at minimum deviation position of prism.
A. The angle of prism becomes zero.
B. Angle of refraction at first refracting surface \( r_1 \) is equal to angle of refraction at second refracting surface \( r_2 \).
C. The refracted ray inside the prism is parallel to the base of the prism.
D. Angle of emergence becomes \( 90^\circ \).
E. Angle of incidence is equal to angle of emergence.
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → B, C and E only
At minimum deviation the ray passes through the prism symmetrically: the path is the same whether the light goes forward or backward, so the angles at the two faces are equal.
\( i = e \)
\( r_1 = r_2 = A/2 \)
A. The angle of prism becomes zero. — Incorrect. The angle of the prism \( A \) is fixed by its shape; it does not depend on how the ray passes. It is related to the refraction angles by \( r_1 + r_2 = A \).
B. Angle of refraction at first refracting surface \( r_1 \) is equal to angle of refraction at second refracting surface \( r_2 \). — Correct. By symmetry \( r_1 = r_2 = A/2 \).
C. The refracted ray inside the prism is parallel to the base of the prism. — Correct. Equal angles at the two equally inclined faces make the ray inside the prism run parallel to the base.
D. Angle of emergence becomes \( 90^\circ \). — Incorrect. At minimum deviation the angle of emergence equals the angle of incidence; \( e = 90^\circ \) is the grazing-emergence case, not minimum deviation.
E. Angle of incidence is equal to angle of emergence. — Correct. This is the condition \( i = e \) for minimum deviation.
NCERT (2026-27):
Refraction Through a Prism – “… At the minimum deviation Dm, the refracted ray inside the prism becomes parallel to its base. We have δ = Dm, i = e which implies r1 = r2.”