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

The light escape is confined within a cone of apex angle '2θc ' where c θc is the critical angle. Imagine a sphere with source of light as its centre and the surface area abc is A.A=∫0θc 2πR2sin⁡θdθ=2πR21−cos⁡θc=πR2 ∴θc=sin−1⁡32=60∘∴ Power transfer =P×A4πR2=4×14=1W