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Question

A point source of power 4W is placed 1m below the free surface of liquid whose refractive index is 23Find the rate of transfer of energy (in watt) from the liquid surface to air. Ignore any absorption or scattering of light energy.

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Solution


The light escape is confined within a cone of apex angle 2θcwhere θcis the critical angle. Imagine a sphere with source of light as its centre and the surface area abc is A. Here A = θc02πR2sinθdθ=2πR2(1cosθc) = πR2[θc=sin1(32=60)] Power transfer = P ×A4πR2 = 4 ×14 = 1 W

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