Why prisms disperse light into seven colours?

Why prisms disperse light into seven colours?

PROF WAQAR HUSSAIN

        

         When white light passes through a prism, the different colours are refracted differently. Light of particular colour is refracted in a direction  different than light of any other colour As the refraction of light of each colour is different, so white light after passing through a prism is separated into different colours. Separation of white light into its constituent colours is called dispersion of light.

        We know that the speed of light changes as it passes from one medium to another. The decrease in the speed of the violet light is maximum when it passes from air to glass, while the speed of red light is decreased the least. That is why the violet light is bent the most and red light the least.

        

         Red light has the largest wavelength (the distance between two adjacent crests and troughs) and the violet light has the smallest wavelength. On passing through a prism, a radiation of longer wavelength is bent to a smaller degree and radiation of smaller wavelength is bent to a greater degree. Thus every colour bends differently according to its wavelength and separate. Let’s have an analogous example, red has large wavelength like the stride of a large man and violet has a small wavelength like a small kid. When, red light and violet light pass through a curved hurdle of prism; red bends the least and violet the most. Likewise if a brick is placed on the way of man and kid, the large stride man will experience fewer hurdles , his speed will reduce a little and thus his walking posture will bend slightly as compared to small stride kid who will experience greater hurdle, his speed will reduce more and his walking mode will bend greatly. Similarly long stride red bends less and short stride violet bends too much.