State the cause of dispersion of white light by a glass prism. How did Newton, using two identical glass prisms, show that white light is made of seven colours? Draw a ray diagram to show the path of a narrow beam of white light, through a combination of two identical prisms arranged together in inverted position with respect to each other, when it is allowed to fall obliquely on one of the faces of the first prism of the combination.

Q: State the cause of dispersion of white light by a glass prism. How did Newton, using two identical glass prisms, show that white light is made of seven colours? Draw a ray diagram to show the path of a narrow beam of white light, through a combination of two identical prisms arranged together in inverted position with respect to each other, when it is allowed to fall obliquely on one of the faces of the first prism of the combination.

### Cause of Dispersion of White Light White light consists of seven constituent colours (VIBGYOR). When a beam of white light passes through a glass prism, different colours travel with different speeds in the glass medium. Since the refractive index of glass is different for different wavelengths (colours) of light, each colour bends through a different angle with respect to the incident ray. Red light travels the fastest and bends the least, while violet light travels the slowest and bends the most. This variation in bending angles causes the white light to split into its constituent spectrum. ### Newton's Recombination Experiment Sir Isaac Newton proved that white light is made of seven colours using two identical glass prisms ($P_1$ and $P_2$): 1. He kept the first prism $P_1$ in an upright position and allowed a narrow beam of white light to fall obliquely on one of its faces. The prism dispersed the white light into a band of seven colours (spectrum). 2. He then placed the second identical prism $P_2$ in an inverted position relative to the first prism. 3. He allowed all the colours of the spectrum from the first prism to fall on the second inverted prism. 4. He observed that a beam of ordinary white light emerged from the other side of the second prism. This recombination demonstrated that white light is composed of seven colours. ### Ray Diagram ``` Upright Prism (P1) Inverted Prism (P2) /\ ______ / \ \ / / \ --- Red Light ---> \ / White / \ \ / ---> Recombined Light ----> \ \/ White Light \ \ --- Violet Light -> / \________\ / / ```

The Human Eye and the Colourful World

Dispersion of White Light by a Glass Prism

Official Answer

Cause of Dispersion of White Light

White light consists of seven constituent colours (VIBGYOR). When a beam of white light passes through a glass prism, different colours travel with different speeds in the glass medium. Since the refractive index of glass is different for different wavelengths (colours) of light, each colour bends through a different angle with respect to the incident ray. Red light travels the fastest and bends the least, while violet light travels the slowest and bends the most. This variation in bending angles causes the white light to split into its constituent spectrum.

Newton's Recombination Experiment

Sir Isaac Newton proved that white light is made of seven colours using two identical glass prisms ( $P_1$ and $P_2$ ):

He kept the first prism $P_1$ in an upright position and allowed a narrow beam of white light to fall obliquely on one of its faces. The prism dispersed the white light into a band of seven colours (spectrum).
He then placed the second identical prism $P_2$ in an inverted position relative to the first prism.
He allowed all the colours of the spectrum from the first prism to fall on the second inverted prism.
He observed that a beam of ordinary white light emerged from the other side of the second prism. This recombination demonstrated that white light is composed of seven colours.

Ray Diagram


               Upright Prism (P1)            Inverted Prism (P2)
                     /\                             
                    /  \                           \      /
                   /    \     --- Red Light --->    \    /
       White      /      \                           \  /  ---> Recombined
       Light ---->        \                           \/        White Light
                  \        \  --- Violet Light ->     /
                   \__\                        /
                                                    /

dispersionrefractive indexdifferent speedsrecombinationinverted prismspectrumwhite light

Marking Scheme

11 mark
21 mark
31 mark

Hint

Recall that different colours of light have different wavelengths and travel at different speeds in glass. For Newton's experiment, the second prism must be placed upside down (inverted) relative to the first.

Quick Oral Answer

Q: Why does red light bend the least when passing through a prism? A: Red light has the longest wavelength in the visible spectrum, meaning it travels faster in glass compared to other visible colours, resulting in the least deviation (bending).

Analysis & Explanation

Newton's experiment was crucial because it disproved the prevailing belief that the prism itself colored the light. By using a second inverted prism to recombine the spectrum back into white light, he proved that the prism merely separates the constituent colors already present in white light. The dispersion occurs due to the wavelength-dependent refractive index of the glass medium ( $n_{\text{violet}} > n_{\text{red}}$ ).

Common Mistakes

1Students often forget to draw the second prism in an inverted position.
2Confusing which colour bends the most (violet bends the most, red bends the least).
3Not labeling the incident white light, the dispersed spectrum, and the emergent white light.

Interesting Facts

Rainbows are natural examples of dispersion where tiny water droplets act as miniature prisms, dispersing and internally reflecting sunlight.

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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 of White Light by a Glass Prism" 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.