Let two optical mediums of R.I.n1 and n2 respectively be such that n1 > n2 . In the case when angle of incidence is θ1cr (Fig.), from the law of refraction
In the case , when the angle of incidence is θ1 , from the law of refraction at the interface of mediums 1 and 2 .
n1 sin θ1 = n2 sin θ2
But in accordance with the problem