The students in a class took a thick sheet of cardboard and made a small hole in its centre. Sunlight was allowed to fall on this small hole and they obtained a narrow beam of white light. A glass prism was taken and this white light was allowed to fall on one of its faces. The prism was turned slowly until the light that comes out of the opposite face of the prism appeared on the nearby screen. They studied this beautiful band of light and concluded that it is a spectrum of white light.
Read the source and answer the questions that follow.
Generated by claude-sonnet-4-6 · 2026-06-15 06:53 · grounding stimulus
Model Answer
(i) A rainbow formed in the sky after rain is one natural instance where a spectrum of white light (dispersion) is observed.
(ii) White light splits into its seven constituent colours — Violet, Indigo, Blue, Green, Yellow, Orange, and Red (VIBGYOR) — on passing through the prism. This phenomenon is called dispersion of light.
(iii) Two conditions necessary to observe a rainbow:
- The sun should be shining (presence of sunlight) and it should be at the observer's back.
- Rain should be falling (water droplets must be present in the atmosphere) in front of the observer.
Source: Light – Reflection and Refraction / Human Eye and the Colourful World, NCERT Science Class 10
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Explanation
- (i) Any valid natural example of dispersion counts — rainbow is the most expected answer.
- (ii) Examiners want the term dispersion and the mention of VIBGYOR. Both together earn the mark.
- (iii) Two distinct, correctly stated conditions are needed for full 2 marks. The key points are: sunlight behind the observer + rain/water droplets in front. Avoid vague answers like "it should rain and be sunny."