What is “dispersion of white light”? Draw a labelled diagram to illustrate the recombination of the spectrum of white light. Why is it essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other?

Q: What is “dispersion of white light”? Draw a labelled diagram to illustrate the recombination of the spectrum of white light. Why is it essential that the two prisms used for the purpose should be identical and placed in an inverted position with respect to each other?

### Dispersion of White Light The splitting of white light into its component colours (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a glass prism is called **dispersion**. ### Recombination of the Spectrum of White Light Below is the diagram showing the recombination of the spectrum of white light using two identical prisms, one upright ($P_1$) and one inverted ($P_2$): ```diagram {"xrange": [-16.8, 466.8], "yrange": [-152.4, -127.6], "lines": [{"from": [30, -150], "to": [125, -140], "label": "White light"}, {"from": [125, -140], "to": [175, -130], "label": "Spectrum (Red to Violet)"}, {"from": [175, -130], "to": [275, -150], "label": ""}, {"from": [325, -140], "to": [420, -150], "label": "Recombined White light"}]} ``` ### Why Identical and Inverted Prisms are Essential: 1. **Identical Prisms:** The second prism must be identical to the first so that it has the same refracting angle and refractive index. This ensures that the magnitude of dispersion (the degree of bending/separation of different wavelengths) produced by the second prism is exactly equal to that produced by the first prism. 2. **Inverted Position:** Placing the second prism in an inverted position ensures that the refraction/bending of light rays occurs in the opposite direction. While the first prism disperses the white light by bending different colours through different angles (violet the most, red the least), the inverted second prism bends these separated colours in the opposite direction by an equal amount, bringing them back together to emerge as a single, parallel beam of white light.

The Human Eye and the Colourful World

Dispersion and Recombination of White Light

Official Answer

Dispersion of White Light

The splitting of white light into its component colours (VIBGYOR: Violet, Indigo, Blue, Green, Yellow, Orange, Red) when it passes through a glass prism is called dispersion.

Recombination of the Spectrum of White Light

Below is the diagram showing the recombination of the spectrum of white light using two identical prisms, one upright ( $P_1$ ) and one inverted ( $P_2$ ):

Why Identical and Inverted Prisms are Essential:

Identical Prisms: The second prism must be identical to the first so that it has the same refracting angle and refractive index. This ensures that the magnitude of dispersion (the degree of bending/separation of different wavelengths) produced by the second prism is exactly equal to that produced by the first prism.
Inverted Position: Placing the second prism in an inverted position ensures that the refraction/bending of light rays occurs in the opposite direction. While the first prism disperses the white light by bending different colours through different angles (violet the most, red the least), the inverted second prism bends these separated colours in the opposite direction by an equal amount, bringing them back together to emerge as a single, parallel beam of white light.

dispersionsplitting of white lightrecombinationidentical prisminverted positionequal and opposite bending

Marking Scheme

1Definition of dispersion of white light (1 mark)
2Neat, labelled diagram showing recombination of spectrum using two prisms (one upright, one inverted) (1 mark)
3Explanation of why the prisms must be identical (equal dispersion magnitude) and inverted (opposite direction of bending) (1 mark)

Hint

Recall how Isaac Newton proved that sunlight is made of seven colours using a second prism placed upside down.

Quick Oral Answer

Dispersion is the splitting of white light into its seven component colours when passing through a prism. We can recombine these colours by placing an identical prism in an inverted position next to the first one, which bends the separated rays back together into a single white light beam.

Analysis & Explanation

Isaac Newton demonstrated that white light is composed of seven colours by using a two-prism setup. The first prism separates the constituent wavelengths of white light because different colours travel at different speeds in glass, causing them to bend at different angles. The second identical but inverted prism reverses this angular deviation, recombining the divergent coloured rays back into a single beam of white light.

Common Mistakes

1Drawing both prisms in the same upright orientation, which would cause further dispersion instead of recombination.
2Forgetting to label the incident white light, the intermediate spectrum, and the emergent recombined white light.

Interesting Facts

Isaac Newton originally tried to split the individual colours of the spectrum further using a second upright prism, but found that no further splitting occurred, proving that the colours themselves are fundamental and the prism is merely a separator.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2017?

This question carries 3 marks in the CBSE Class 10 Science 2017 examination.

Which chapter does this question come from in Science?

This question is from the chapter "The Human Eye and the Colourful World" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Dispersion and Recombination of White Light" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2017 paper?

This is a Short Answer question from Section A in the CBSE Class 10 Science 2017 paper.