In an experiment to trace the path of a ray of light through a glass prism for different values of angle of incidence a student would find that the emergent ray:

According to the NCERT textbook, the peculiar shape of a triangular glass prism (where the refracting surfaces are inclined to each other at an angle) makes the emergent ray bend at an angle to the direction of the incident ray. This angle between the direction of the incident ray and the emergent ray is called the angle of deviation ($\angle D$).

• Why Option D is correct: The non-parallel lateral faces of the prism cause the light ray to bend at both refracting surfaces such that the emergent ray is inclined at an angle to the original direction of the incident ray.
• Why Option A is incorrect: The emergent ray is parallel to the incident ray (with a slight lateral displacement) only in a rectangular glass slab because its refracting surfaces are parallel. In a prism, the refracting surfaces are inclined, so they are not parallel.
• Why Option B is incorrect: The emergent ray does not bend at a perpendicular angle ($90^\circ$) to the incident ray under normal experimental conditions.
• Why Option C is incorrect: The refracted ray travels inside the prism, while the emergent ray travels outside the prism after undergoing a second refraction. They are not parallel to each other.