**Let two optical mediums of R.I.n**_{1} and n_{2} respectively be such that n_{1} > n_{2} . In the case when angle of incidence is θ_{1cr} (Fig.), from the law of refraction

**In the case , when the angle of incidence is θ**_{1} , from the law of refraction at the interface of mediums 1 and 2 .

**n**_{1} sin θ_{1} = n_{2} sin θ_{}_{2}

**But in accordance with the problem **