State the cause of dispersion of white light by 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 combination of two identical prisms (where one of the prism is in an inverted position with respect to the other), when it is allowed to fall obliquely on one of the face of the first prism of the combination.

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Solution

When a beam of white light enters the prism, it gets refracted at that point and splits into seven constituent colours. This splitting of the light occurs because different colours of light bend through different angles with respect to the incident ray, as they pass through the prism.
Firstly, Newton made white light to fall on a prism. This caused dispersion of white light into seven colours. Newton then placed an inverted prism in the path of the colour band of seven colours. Only a beam of white light came out from the second prism. So, Newton concluded that white light comprises of seven component colours.

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