A ray of red light passes through a glass block, as shown in Figure.
(i) On Figure, clearly label the angle of refraction.
(ii) State the name given to the dashed line drawn at $90^{\circ }$ to the boundary.

(a) Prove that a ray of light incident on the surface of a sheet of plate glass of thickness  $t$  emerges from the opposite face parallel to its initial direction but displaced sideways,as in above figure. (b) Show that, for small angles of incidence  $\theta$, this displacement is given by  $x=t\theta \frac{n-1}{n}$.
where  $n$  is the index of refraction of the glass and  $\theta$  is measured in radians.