A glass prism of refractive index 1.56 is immersed in water (R.I. = 4/3). The beam of light incident normally on the face AB is totally reflected to reach the face BC, if

None of these

The light ray is just reflected internally at the point P (say). Therefore the critical angle ≅ θ. Applying Snell’s Law of refraction at the critical angle of incidence (θ) we obtain

$\frac{\sin θ}{\sin 90}=\frac{n_w}{n_g}⇒\sin θ=\frac{4/3}{3/2}=\frac{8}{9}$

$⇒θ≤\sin^{-1}\frac{8}{9}$