Physics
Optical fibres are based on the phenomenon of

Interference
Diffraction
Dispersion
Total Internal Reflection

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Physics
In total internal reflection, the light travel from

Denser to rarer medium and it occurs with no loss of intensity
Denser to rarer medium and it occurs with loss of intensity
Rarer to denser medium and it occurs with no loss of intensity
Rarer to denser medium and it occurs with loss of intensity

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