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When a light ray is incident from a denser medium Mu1 to rarer medium Mu2 (<Mu1) at angle of incidence i, the angle of refraction is given by :

Sin i / Sin r = Mu2/Mu1
Sin i = ( Mu2 / Mu1 ) Sin r

here r > i as Mu1 > Mu2.

As i increases r increases. At i = i_critical angle, the r becomes 90 deg and the light ray grazes (goes along) the boundary between two media.

For i > i_critical, the light ray is totally reflected in to the medium1.