Case: Read the passage and answer the question.
Total internal reflection is the reflection of light back into the denser medium at its interface with a rarer medium. For it to occur, light must travel from the denser medium towards the rarer medium, and the angle of incidence in the denser medium must be greater than the critical angle for the pair of media. The critical angle depends on the nature of the media and on the wavelength of light.
Which phenomenon is used in optical fibres?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Total internal reflection
Option (1) Reflection — Incorrect. At an ordinary reflecting surface some of the light is always transmitted, so the reflected ray is weaker than the incident ray. After the many reflections along a long fibre, very little light would be left.
Option (2) Diffraction — Incorrect. Diffraction is the bending and spreading of light around the edges of an obstacle or an aperture. It does not keep light confined inside a fibre.
Option (3) Total internal reflection — Correct. An optical fibre has a core of higher refractive index than its cladding. Light sent in at one end meets the core-cladding boundary. There it goes from the denser to the rarer medium at an angle greater than the critical angle. So it is totally reflected again and again. It comes out at the other end with very little loss, even when the fibre is bent.
NCERT (2026-27):
Total Internal Reflection in Nature and Its Technological Applications – “(ii) Optical fibres: Nowadays optical fibres are extensively used for transmitting audio and video signals through long distances. Optical fibres too make use of the phenomenon of total internal reflection.”