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Dispersion of White Light -by-  a Prism

Sir Isaac Newton was the first to carry out a systematic study of refraction of light through a prism. While studying the images of heavenly bodies seen through a telescope, he found that the edges of the images were colored. He allowed sunlight to enter his darkened room through a small hole in a window and placed a prism in the path of the rays as shown. He received the light emerging from the prism on a white screen. He then expected to see a single white image of the sun on the screen, but instead, he saw a band of colours of a rainbow.


Starting from the side of the base of the prism, he saw seven colours in the following order.

This coloured band is called the spectrum and this phenomenon of white light splitting into its constituent colours is called dispersion.

In order to show that the prism did not produce the colours, but merely separated the colours already existing in the white light of the sun, Newton did a second experiment. He made a small hole in the screen so that light of just one colour could pass through it at a time and then arranged a second prism. When he examined each colour in turn emerging from the second prism, he found that there was no further splitting and that the rays of different colours simply bent on passing through the second prism. He made a very important observation. He observed that different colours bend by different amounts on passing through the prism. The violet colour deviated the most and red, the least.


Explanation of Dispersion

From his experimental observations, Newton concluded that white light consists of a mixture of seven different colours. The refractive index of glass is different for different colours, so that when white light falls on glass prism, each colour is refracted (or bent) at a different angle. The colours spread out to form a spectrum. All colours are refracted towards the base of the prism, the violet being deviated the most and red the least. The splitting up of white light into its constituent colours is, called dispersion. Each colour has its own characteristic wavelength. 



Recombination of Spectral Colours

Newton argued that if white light is a mixture of seven different colours, it should be possible to recombine these colours to produce white light. To demonstrate this, he took two prisms of the same material and of the same refracting angle A and arranged them as shown. The first prism splits the sunlight into the seven colours (VIBGYOR). The spectral colours emerging from this prism are allowed to fall on another prism placed with its base upwards (upright) so that this prism refracts light in a direction opposite to the first. The light emerging from the second prism is found to be white, clearly establishing that white light indeed consists of seven different colours.


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