Optical fibers transmit light by total internal reflection.

Solution

Physics sir · 4 steps

Light stays trapped in the fibre because every time it hits the wall, it is reflected back completely.

1. Fibre structure. An optical fibre has a glass core with a higher refractive index n₁. It is wrapped in a cladding with a slightly lower refractive index n₂.
2. Two conditions. Total internal reflection needs light going from a denser to a rarer medium, which is true since n₁ > n₂. It also needs the angle of incidence to be greater than the critical angle C, given below.

   sin C = n₂/n₁

3. Bouncing along. Light enters the core at a small angle to the axis, so it strikes the core–cladding boundary at a large angle, more than C. Each time it is totally reflected, with no light refracted out, so it zig-zags along the fibre with very little loss, even around gentle bends.
4. So. Optical fibres guide light by repeated total internal reflection. The statement is true.

The idea to keep. For total internal reflection, check two things: light must go from denser to rarer, and the angle of incidence must exceed the critical angle. Fibres, mirages and sparkling diamonds all use it.

Answer from the NTA final answer key. Solution by PrepWiser.