For    T.I.R. at A

$4sin30^{\circ }={\mu }_{2}sin90^{\circ }$

${\mu }_2=2$

Case -I

If ${\mu }_2<2$, then always T.I.R. takes place and in this situation angle of deviation is 120^o.



Case-II

${\mu }_2>2$, then for the given angle of incidence no. T.I.R takes place at A, so light strikes on interface B for T.I.R.



${\mu }_{2}sin\alpha =2sin90^{\circ }$

or     ${\mu }_2=\frac{2}{sin\alpha }$

$sin\alpha <1$

${\mu }_2>2$