A rectangular block of glass ABCD has a refractive index 1.6. A pin is placed midway on the face AB figure. When observed from the face AD, the pin shall

For $\mu = 1.6$, the critical angle, $\mu = 1/\sin C$, we have $C = 38.7°$, when viewed from AD, as long as angle of incidence on AD of the ray emanating from pin is greater than the critical angle, the light suffers from total internal reflection and cannot be seen through AD. For any ray ,emerging from the pin incident on the face AB , the refracted ray is incident on the surface AD at an angle of incidence greater than ${45}^0$and this ray will suffer total internal reflection .