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Optical fibres use TOTAL INTERNAL REFLECTION to transmit light over long distances. The MAIN condition for the fibre to work is
Athe core's refractive index must be LESS than the cladding's
Bthe core's refractive index must be GREATER than the cladding's
Cthe fibre must be coated with a metallic mirror outside
Dthe fibre must be filled with air
Answer & Solution
Correct answer: B. the core's refractive index must be GREATER than the cladding's
1. NCERT §9.4.1 (Optical Fibres) explains: for TIR to occur, light must travel from a DENSER medium to a RARER one.
2. In an optical fibre, the CENTRAL CORE is the denser medium ($n_\text{core}$ higher) and the surrounding CLADDING is the rarer medium ($n_\text{cladding}$ lower).
3. At each reflection at the core-cladding boundary, if the angle of incidence exceeds the critical angle $\sin^{-1}(n_\text{cladding}/n_\text{core})$, the light TOTALLY reflects back into the core.
4. As the fibre bends, light zigzags down the length with negligible loss. Result: signals propagate over kilometres with very low attenuation — the backbone of modern telecommunications.
5. Option A reverses the condition (TIR couldn't occur). Options C and D have no physical basis.
_Source: NCERT Class 12 Physics Part 2, Ch 9, §9.4.1 (Total Internal Reflection in Optical Fibres), p. 8._
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