The phenomenon used in optical fibre for transmission of light energy is

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UnlockIAS · Accepted Answer

Correct — A, Total internal reflection. An optical fibre is a thin thread of glass or plastic built in two layers: an inner core of higher refractive index and an outer cladding of lower refractive index. Light that enters the fibre within its acceptance cone strikes the core-cladding boundary at an angle greater than the critical angle for that pair of media. At such angles no light is refracted out into the cladding; the entire beam is thrown back into the core. Repeating this thousands of times, the signal zig-zags along the fibre — even round bends — with very little loss, which is why fibre carries telephone and internet traffic across continents and under oceans. The confinement is total, and that is precisely what the word 'total' in total internal reflection means: unlike an ordinary mirror, which absorbs a few per cent at every bounce, this reflection returns all the light.

Why the other options are wrong — (b) Diffraction is the bending and spreading of waves as they pass an obstacle or a narrow opening — it explains the coloured bands on a CD, the pattern from a grating and the limit on a telescope's resolving power. It plays no part in guiding light down a fibre.  (c) Scattering is the redirection of light by particles or molecular irregularities, as in the blue sky and the red sunset. In a fibre scattering is actually the enemy: residual scattering inside the glass is one of the main causes of signal loss, so fibres are manufactured to minimise it, not to rely on it.  (d) This is the intended trap, and it is half-right for the wrong reason. Refraction — the bending of light when it crosses between media of different refractive index — is what happens as light enters the fibre's end face, and the difference in refractive index between core and cladding is what makes total internal reflection possible at all. But if light were merely refracted at the core-cladding wall it would pass out of the fibre and be lost. Transmission along the fibre works because refraction is prevented and the light is totally reflected.

Concept: When light travels from an optically denser medium to a rarer one, it bends away from the normal. Increase the angle of incidence and the refracted ray bends further, until at one particular angle — the critical angle — it grazes along the boundary itself. Beyond that angle, refraction is impossible and all the light is reflected back into the denser medium. That is total internal reflection, and it requires two conditions together: the light must be going from denser to rarer, and the angle of incidence must exceed the critical angle. The critical angle depends on the pair of media: roughly 42 degrees for ordinary glass to air, about 48.6 degrees for water to air, and only about 24.4 degrees for diamond to air, which is why a cut diamond traps and returns so much of the light entering it.

State PCS 2019 Q90 — Science & Technology PYQ with Answer

State PCS 2019 Q90 — Science & Technology PYQ with Answer