Optical fibre works on the principle of

Concept

Total internal reflection (TIR) occurs when a light ray travelling inside an optically denser medium strikes the boundary with a rarer medium at an angle of incidence greater than the critical angle for that pair of media. Instead of refracting out into the rarer medium, the entire ray reflects back into the denser medium — no light energy crosses the boundary.

Application

1. An optical fibre has a core with refractive index n1 surrounded by a cladding with a lower refractive index n2 (so n1 > n2).

2. Light launched into the core strikes the core–cladding boundary at an angle greater than the critical angle for the n1–n2 pair.

3. By total internal reflection, this light is reflected back into the core with no loss through the boundary, instead of leaking out through the cladding.

4. The ray keeps striking the boundary at the same angle at every subsequent bounce, so it undergoes total internal reflection repeatedly and is guided along the entire length of the fibre with very low loss.

Cross-check

Contrasting with the other named phenomena confirms this: ordinary reflection can be partial — it does not require the incidence angle to exceed a critical angle, so some light would leak out at every bounce instead of being fully guided. Diffraction is the spreading of a wave around an obstacle or through a narrow aperture, governed by wavelength versus aperture size, and plays no role in confining a ray inside a boundary. Polarisation restricts the plane in which the light wave oscillates and does not by itself confine or guide a ray along a path. Only total internal reflection supplies the critical-angle condition needed for a ray to be captured and guided along the fibre with minimal loss.