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Question

When a ray of light passes through a triangular glass prism, the emergent ray is generally not parallel to the incident ray. Which of the following best explains this observation?

This question was previously asked in
RRB Group D 2025 Question Paper (18-Aug-2026) (Shift 1)
The correct answer is

The two refracting faces of the prism are inclined to each other, causing an overall deviation of the light ray.

A triangular prism has two refracting surfaces that are inclined to each other at an angle called the angle of prism, unlike a rectangular glass slab whose faces are parallel.

Light bends towards the normal while entering the denser prism and away from the normal while leaving it, and because the two surfaces are not parallel, the net effect is a permanent deviation of the ray.

This is why the emergent ray from a prism is not parallel to the incident ray, unlike in a glass slab where the emergent ray is parallel but laterally shifted.

Hence, the inclination of the two refracting faces of the prism causes the overall deviation of the light ray.

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