JEE Main · 2014 · 06 Apr 2014 · Offline · Geometrical Optics

When white light meets the water-to-air boundary, each colour has its own refractive index, and the higher the frequency the larger that index, so the critical angle ${\theta }_c={\sin }^{-1}(1/\mu )$ is smallest for the high-frequency colours. Green is set exactly at its critical angle here. Any colour of higher frequency than green, such as blue and violet, has an even smaller critical angle, so green's angle already exceeds it and those colours are turned straight back into the water by total internal reflection. The colours of lower frequency than green, the yellow, orange and red, have a larger critical angle, so their angle of incidence falls short of it and they pass out into the air. Only the part of the visible spectrum with frequency less than that of green emerges into the air.