The angle of incidence corresponding to an angle of refraction 90°,
say ÐAO3N, is called the critical angle (ic ) for the given pair of media We
see from Snell’s law [Eq (9 10)] that if the relative refractive index of the
refracting medium is less than one then, since the maximum value of sin
r is unity, there is an upper limit to the value of sin i for which the law
can be satisfied, that is, i = ic such that
sin ic  = n 21
(9