Fish trick
A ray of light passing from denser to rarer medium can be totally internally reflected only if angle of incidence becomes greater than the critical angle the converse process is not true.
the best of it can be imagined as follow -
a fish looking from within water sees the outside world through a circular horizon if fish is (7^1/2) cm below the surface of water . the radius of horizon would be
(refractive index of water with respect to air =4/3)
The answer is 3.
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1 solution
From 1/Refractive Index = r/sqrt(r^2 + d^2) => 3/4 = r/(sqrt(r^2 + 7)) (Refractive Index = 4/3, depth d = 7^(1/2)). r = 3 cm.