For total internal reflection to occur, which of the following is correct?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Light travels from a denser to a rarer medium.
Option (1) Light travels from a rarer to a denser medium. — Incorrect. Entering a denser medium, the ray bends towards the normal, so the angle of refraction is always smaller than the angle of incidence. A refracted ray exists for every angle of incidence, and the light is never totally reflected.
Option (2) Light travels from a denser to a rarer medium. — Correct. Going into a rarer medium, the ray bends away from the normal. The angle of refraction grows with the angle of incidence until it reaches \( 90^\circ \) at the critical angle. For larger angles of incidence refraction is not possible, and the light is totally reflected back into the denser medium.
Option (3) Light travels in air only. — Incorrect. Total internal reflection happens at the boundary between two media. Light that stays in air alone meets no such boundary to be reflected from.
Option (4) Light travels in water only. — Incorrect. Light inside water is totally reflected only where it meets the boundary with a rarer medium such as air, at more than the critical angle. Within water alone, it just travels on.
NCERT (2026-27):
Total Internal Reflection – “When a ray of light enters from a denser medium to a rarer medium, it bends away from the normal, … If the angle of incidence is increased still further …, refraction is not possible, and the incident ray is totally reflected. This is called total internal reflection.”