Optical fiber uses reflection to guide light through a channel, in which angle…

Answer: greater than

Explanation: Optical fibers guide light by total internal reflection at the boundary between the core (higher refractive index) and the cladding (lower refractive index).

• Key condition: total internal reflection happens when light tries to pass from a medium with higher refractive index to one with lower refractive index and the angle of incidence is greater than the critical angle.

• Critical angle formula: sin θc = n_cladding / n_core (valid when n_core > n_cladding).

• If the angle equals the critical angle, the refracted ray travels along the interface (emerges at 90°) and is not totally reflected back into the core.

• If the angle is less than the critical angle, part of the light is transmitted into the cladding and the ray is not confined by reflection.

• Therefore, to ensure light is guided by reflection inside the fiber, the angle of incidence at the core–cladding boundary must be greater than the critical angle.

Example: For a core index 1.50 and cladding index 1.40, sin θc = 1.40/1.50 = 0.9333, so θc ≈ 69.1°. Any incidence angle greater than about 69.1° (measured from the normal) will undergo total internal reflection.