The critical angle of incidence \( i_c \) for a ray incident from a denser to rarer medium, is that angle for which
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → the angle of refraction is \( 90^\circ \)
When light goes from a denser to a rarer medium, the ray bends away from the normal. The critical angle is the angle of incidence for which the refracted ray just grazes the surface, so the angle of refraction is \( 90^\circ \). Snell's law gives
\( \displaystyle n_1\sin i_c = n_2\sin 90^\circ \;\Rightarrow\; \sin i_c = \frac{n_2}{n_1} \)
Option (1) the angle of reflection is \( 90^\circ \) — Incorrect. The angle of reflection equals the angle of incidence, which is \( i_c \), and this is less than \( 90^\circ \).
Option (2) the angle of refraction is \( 90^\circ \) — Correct. At the critical angle the refracted ray travels along the boundary.
Option (3) the angle of refraction is \( 0^\circ \) — Incorrect. The angle of refraction is \( 0^\circ \) only for normal incidence.
Option (4) the angle of reflection is \( 0^\circ \) — Incorrect. The angle of reflection is \( 0^\circ \) only for normal incidence.
NCERT (2026-27):
Total Internal Reflection – “… The angle of incidence corresponding to an angle of refraction 90°, say ∠AO3N, is called the critical angle (ic) for the given pair of media.”