(a) Draw the ray diagram for refraction of light through a glass prism and mark the angle of refraction and the angle of deviation.

(b) When the path of a light ray refracted through a glass prism is reversed, how will the angle of deviation change? Explain.

Section 9.3.1 (Activity 9.10) of the chapter describes exactly this situation: 'The light ray at point O has entered from a rarer medium to a denser medium, that is, from air to glass. Note that the light ray has bent towards the normal. At O′, the light ray has entered from glass to air, that is, from a denser medium to a rarer medium. The light here has bent away from the normal.'

The chapter further states: 'In Fig. 9.10, a ray EO is obliquely incident on surface AB, called incident ray. OO′ is the refracted ray and O′H is the emergent ray. You may observe that the emergent ray is parallel to the direction of the incident ray... The extent of bending of the ray of light at the opposite parallel faces AB (air-glass interface) and CD (glass-air interface) of the rectangular glass slab is equal and opposite. This is why the ray emerges parallel to the incident ray. However, the light ray is shifted sideward slightly.'

This follows from the laws of refraction (Section 9.3.1): at each surface the incident ray, refracted ray and normal lie in the same plane, and $\dfrac{\sin i}{\sin r}=\text{constant}$ (Snell's law, Eq. 9.4) for a given pair of media. Since face AB and face CD are parallel, the normal at O and the normal at O′ are also parallel; going from air to glass at O bends the ray by a certain angle towards the normal, and going from glass to air at O′ (the reverse pair of media, same refractive index ratio) bends it back by the exact same angle away from the normal — so the net direction is unchanged, only the ray's position is shifted sideways (the lateral displacement).