How will you use two identical glass prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw and label the ray diagram.
How will you use two identical glass prisms so that a narrow beam of white light incident on one prism emerges out of the second prism as white light? Draw and label the ray diagram.
Official answer
To obtain white light from the second prism after dispersion from the first, we must place a second identical glass prism in an inverted position with respect to the first prism.
- Dispersion: The first upright prism () disperses the incident narrow beam of white light into its seven constituent colours (spectrum), where red light bends the least and violet light bends the most.
- Recombination: The second inverted prism () receives this dispersed spectrum. Because of its inverted orientation, it bends the different colours in the opposite direction by equal amounts, recombining them into a single emergent beam of white light.
``diagram {"xrange": [-12.0, 422.0], "yrange": [-180.6, -19.4], "lines": [{"from": [30.0, -110.0], "to": [125.0, -100.0], "label": ""}, {"from": [125.0, -100.0], "to": [175.0, -80.0], "label": ""}, {"from": [125.0, -100.0], "to": [175.0, -120.0], "label": ""}, {"from": [175.0, -80.0], "to": [245.0, -80.0], "label": ""}, {"from": [175.0, -120.0], "to": [245.0, -120.0], "label": ""}, {"from": [245.0, -80.0], "to": [295.0, -100.0], "label": ""}, {"from": [245.0, -120.0], "to": [295.0, -100.0], "label": ""}, {"from": [295.0, -100.0], "to": [380.0, -100.0], "label": ""}], "points": [{"x": 150.0, "y": -165.0, "label": ""}, {"x": 270.0, "y": -35.0, "label": ""}, {"x": 50.0, "y": -90.0, "label": ""}, {"x": 210.0, "y": -75.0, "label": ""}, {"x": 210.0, "y": -135.0, "label": ""}, {"x": 350.0, "y": -90.0, "label": ""}], "caption": "Recombination of the Spectrum of White Light"} ``
Marking scheme
- 1.State that the second identical prism must be placed in an inverted position relative to the first. (1 mark)
- 2.Explain the process: the first prism disperses white light, and the second inverted prism recombines the spectrum back into white light. (1 mark)
- 3.Draw a neat, labelled ray diagram showing the incident white light, the dispersed spectrum (Red and Violet rays), and the emergent white light. (1 mark)
Hint
Recall Isaac Newton's recombination experiment. How must the second prism be oriented relative to the first to reverse the dispersion?
Quick oral answer
We place the second identical prism in an inverted position. The first prism disperses the white light, and the second inverted prism recombines the dispersed colors back into white light.
Common mistakes
- 1.Drawing both prisms in the same upright orientation (which would cause further dispersion instead of recombination).
- 2.Not labelling the intermediate rays (Red on top, Violet at the bottom) or the final emergent beam as 'white light'.
Analysis and explanation
This experiment was originally performed by Sir Isaac Newton to prove that white light is composed of seven constituent colours, rather than the prism adding colour to the light. The first prism splits the white light because different wavelengths (colours) of light travel at different speeds in glass, leading to different angles of deviation. By placing an identical second prism in an inverted position, the angular dispersion is exactly reversed, bringing all the separated wavelengths back together into a parallel beam of white light.
Interesting facts
This classic experiment by Isaac Newton debunked the prevailing theory of his time that the glass of the prism itself colored the light. It proved that color is an intrinsic property of light itself.
Question details
Q24 of 30 questions
- Board
- CBSE
- Class
- Class 10
- Subject
- Science
- Year
- 2020
- Section
- Section B
- Marks
- 3
- Type
- Short Answer
- Topic
- Dispersion of White Light by a Glass Prism
- Bloom's level
- Applying
- Syllabus topic
- Dispersion of light through a prism
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Frequently asked questions
How many marks does this question carry in CBSE Class 10 Science 2020?
This question carries 3 marks in the CBSE Class 10 Science 2020 examination.
What topic does this question cover in Science?
This question covers the topic "Dispersion of White Light by a Glass Prism" from CBSE Class 10 Science.
What type of question is this in the CBSE Class 10 Science 2020 paper?
This is a Short Answer question from Section B in the CBSE Class 10 Science 2020 paper.