A glass prism of refractive index 1.5 is immersed in water (µ = 4 /3). A light beam incident normally on the face AB is totally reflected to reach the face BC if

sin θ > 8/9

From the figure, it is clear that TIR takes place at AC, only if θ > C

$\Rightarrow \sin \theta>\sin C \Rightarrow \sin \theta>\frac{1}{{ }_\omega \mu_{\mathrm{g}}}$

$\Rightarrow \sin \theta>\frac{1}{9 / 8} \Rightarrow \sin \theta>\frac{8}{9}$