Total Internal Reflection

Optical fibres, as explained in NCERT, guide light using total internal reflection, which requires the light to travel in a denser core surrounded by a rarer cladding so that rays striking the boundary beyond the critical angle reflect completely. Because no light energy escapes at each reflection, the signal can travel for kilometres with very little loss, bouncing repeatedly along the fibre even around gentle bends. This is fundamentally a refraction-boundary phenomenon governed by the refractive indices of core and cladding, not by the wave-spreading behaviours of the other options. Diffraction is wrong because it describes light bending around obstacles or through apertures, which is not the confinement mechanism here. Dispersion is wrong because it merely splits light into colours and would in fact degrade a signal rather than confine it. Polarization is wrong because aligning oscillation directions does nothing to trap light inside the fibre. As stated in NCERT Class 12, Chapter 9, the cladding has a slightly lower refractive index than the core precisely to ensure total internal reflection. A consistency check confirms that only total internal reflection provides lossless confinement, which is why fibre communication is efficient.