When light passes through a rectangular glass slab, the emergent ray is parallel to the incident ray. When light passes through a triangular glass prism, the emergent ray is NOT parallel to the incident ray. What feature of the prism is responsible for this difference?
Generated by claude-sonnet-4-6 · 2026-06-26 01:08 · grounding rag
Model Answer
In a rectangular glass slab, the two refracting surfaces are parallel to each other. The bending at the two surfaces is equal and opposite, so the emergent ray comes out parallel to the incident ray.
In a triangular glass prism, the two refracting surfaces are inclined to each other (at the angle of the prism). Because of this inclination, the bending at the two surfaces does not cancel out, and the emergent ray is deviated from the direction of the incident ray (angle of deviation ∠D is produced).
Source: Chapter 10, Section 10.3; Chapter 9, Section 9.3.1
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Explanation
- Examiners want you to explicitly contrast parallel surfaces (slab) with inclined surfaces (prism).
- Key phrase from the textbook: "The peculiar shape of the prism makes the emergent ray bend at an angle to the direction of the incident ray."
- Mention angle of deviation for full marks — it shows you understand the consequence of the inclined surfaces.
- Do not write vague answers like "the prism has a different shape" — always state how the shape (inclination of surfaces) causes the deviation.