Optical fibre works on the principle of

Optical fibres guide light by trapping it in a high-refractive-index core surrounded by a lower-index cladding; when a ray in the core strikes the core–cladding interface at an angle greater than the critical angle it undergoes total internal reflection and continues to propagate along the fibre with very low loss. The idea of optical density and refractive index determines which medium is optically denser and hence sets the critical angle for total internal reflection [1]. Technical references on fiber optics explicitly state that an optical fibre confines light via repeated total internal reflections at the core–cladding interface, distinguishing this mechanism from mere refraction, scattering or interference [2].

Sources

1. [1] Science , class X (NCERT 2025 ed.) > Chapter 9: Light – Reflection and Refraction > Activity 9.10 > p. 149
2. [2] https://www.bietdvg.edu/media/department/PHY/data/learning-materials/Optical_fibers_and_dielectrics.pdf