A person allowed a narrow beam of white light from the sun to enter a dark room through a small aperture and placed a glass prism in its path in such a manner that the beam falls on the face AB of the prism as shown in the figure. A screen S is placed on the other side of the prism, facing AC. On turning the prism slowly, a beautiful band of colours is obtained on the screen. It is the spectrum of sunlight.
Read the case carefully and answer the questions that follow.
Generated by claude-sonnet-4-6 · 2026-06-15 06:51 · grounding stimulus
Model Answer
(a) The phenomenon is called dispersion of light.
(b) White light is composed of seven colours (VIBGYOR). Each colour has a different wavelength and travels at a different speed in glass, so the prism bends (refracts) each colour by a different angle, splitting them into a band of seven colours.
(c)(i)
Experimental arrangement for recombination:
Two identical glass prisms P₁ and P₂ are placed with their bases in opposite directions (inverted with respect to each other).
- Prism P₁ disperses white light into seven colours (VIBGYOR).
- Prism P₂ acts as the recombining prism; it refracts each colour in the opposite direction.
- The colours recombine to give back white light on the screen.
Labelled Ray Diagram:
```
White light → [P₁] → VIBGYOR (spectrum) → [P₂ inverted] → White light → Screen
```
(P₁: dispersing prism; P₂: inverted prism; arrows show direction of rays)
Source: Light – Reflection and Refraction / Human Eye and the Colourful World, NCERT Class 10 Science
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Explanation
- (a) "Dispersion" is the one-word answer examiners expect. Do not write "refraction" alone.
- (b) Mention two things: white light has 7 colours + each colour refracts differently due to different wavelength/speed. Both points are needed for full credit.
- (c)(i) The key is the inverted second prism — this is the standard NCERT experiment. Draw a clear diagram with P₁, P₂ (inverted), spectrum between them, and white light emerging. Even a neat schematic diagram with labels earns marks; a missing diagram loses marks.