Science — CBSE Class 10 board question

(a) Ray diagram for refraction through a glass prism:

```
A
/\
/ \
i→ / \
─────/──────\─────
r│ │r'
│ │
─────\──────/──────
\ / →e
\ /
\/
B
```

Draw incident ray, refracted ray inside prism, and emergent ray.

• Angle of refraction (r): angle between the refracted ray inside the prism and the normal at the first surface.
• Angle of deviation (D): angle between the original direction of the incident ray and the direction of the emergent ray.

(b) When the path of the light ray is reversed, the angle of deviation remains the same.

This is because refraction is a reversible process. The reversed ray retraces the original path — it enters at the second surface and exits at the first — but the angle between the incident and emergent directions remains unchanged. The angle of deviation depends only on the angle of incidence and the prism, not on the direction of travel.

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• Part (a) carries ~2 marks: draw a clear labelled diagram showing the incident ray, normal at both surfaces, refracted ray inside the prism, emergent ray, and mark r (angle of refraction at first surface) and D (angle of deviation). Label the prism clearly.
• Part (b) carries ~1 mark: the key fact is that the angle of deviation does not change because refraction is reversible. State the reason briefly.
• Examiners look for the correct labelling in the diagram and the correct conclusion (unchanged) with a one-line reason in part (b).