A narrow beam XY of white light is passing through a glass prism ABC as shown in the diagram. Trace it on your answer sheet and show the path of the emergent beam as observed on the screen PQ. Name the phenomenon observed and state its cause.
Generated by claude-sonnet-4-6 · 2026-06-15 06:54 · grounding rag
Model Answer
Path of emergent beam: The white light entering the prism splits into a band of seven colours — Violet, Indigo, Blue, Green, Yellow, Orange, Red (VIBGYOR) — on the screen PQ. Violet deviates the most and appears at the top (near P), red deviates the least and appears at the bottom (near Q).
Phenomenon: Dispersion of white light.
Cause: Different colours of white light bend through different angles while passing through the prism, because each colour travels at a different speed in glass. Violet bends the most and red the least.
Source: Chapter 10, Section 10.4 – Dispersion of White Light by a Glass Prism
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Explanation
- Examiners expect: (1) a correctly labelled diagram/trace showing the spectrum with VIBGYOR order, (2) the name "dispersion," and (3) the cause (different colours bend by different amounts / travel at different speeds in glass).
- A common mistake is reversing the order — remember violet is most deviated (bends toward the base more), so it appears lower on the screen if the prism apex points up. In the standard diagram with apex at top and screen to the right, violet is at the bottom (toward Q) and red at the top (toward P) — check the prism orientation in your diagram carefully before drawing.
- "VIBGYOR" must appear in your answer to show you know the colour sequence.