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Dispersion of light
Light coming from the sun or electric bulb is called white light. If this light is allowed to fall on a prism it spits up into its constituent colours. Hence “splitting up of white light ito its constituent colours is called dispersion of light”.
Newton’s experiment for dispersion of light
In order to observe spectrum of light Newton performed an experiment by making a small circular hole in one of the window of his room. When sunlight entered in the room through this hole, a spot of white light was formed on the opposite wall. He introduced a prism in the path of narrow beam of white light . Light after passing through the prism formed a pattern of seven colors on the wall.Newton explained the spectrum by the theory that white light consists of a mixture of light of seven colors. The refractive index of glass is different for light of each colour. So that white light falls on the prism each colour in it is refracted at a different angle so the colours are spread out to form a spectrum.
Improvement in Newton’s experiment for dispersion of light
The spectrum formed in Newton’s first experiment was not pure. The spectrum obtained consisted of series of circular coloured images overlapping on each other. Later on Newton deviced the following arrangement .
A converging lens ‘L’ is placed so as to form an image ‘I’ , of a narrow slit ‘S’ which is illuminated by a bulb of white light. A prism is placed between the screen and the lens ‘L’. When light passed through the prism, dispersion is produced and a pure spectrum is seen on a screen. This consists of series of coloured images of the slit, all very close to each other. If slit is made narrow, overlapping is reduced and the resulting spectrum will be more pure. The spectrum will be more clear, if the position of the prism is set for the minimum deviation.
Recombination of colors
The colours of the spectrum may be recombined to form white light by allowing spectrum to be formed on a row of small rectangular plane mirror.
A screen ‘W’ is adjusted at such an angle that all these mirrors reflect the light on the screen at the same point. The rersult is, a spot of white light is formed on the screen.
Newton also performed an experiment regarding the recombination of components colours. He pianted the colours of the spectrum in sectors on a disc.
When it is rotated with high speed, the disc appears white. This happens because of the persistence of vision. the impression of an image on the retina of an eye is retained for a fraction of a second after the image has disappeared. Consequently the brain sums up and mixes together the rapidly changing coloured images of the disc. thus the sensation of the stationary white image is produced.
Rainbow
The rainbow is a natural phenomenon sometimes seen after a rain. In the seventeenth century a French mathematician Rene Descartes gave the idea that rainbow is produced by the dispersion of light. According the Rene, after the rain showers, a number of droplets of water remain suspended high up in the air. When sun light falls on theses water droplets, it gets splitted, as each drop acts like a prism. So each ray of light after passing through the drop suffers refraction and total internal reflection. The violet colour is refracted the most and the red colour, at least. Thus the red ray which emerges from each droplet makes greater angle with the horizontal than the violet ray. Similarly other coloured rays emerge at different angles to the horizontal. The droplets which are at the same angle from the observer will produce the same colour. Consequently an area of coloured bands called the rainbow appear in the sky opposite to the sun as shown in the figure below.
The colour in the rainbow are distributed in such a way that violet lies on the inside of the bow, while the red light outside of the bow.