State the cause of dispersion of white light passing through a glass prism. How did Newton show that white light of Sun contains seven colours using two identical glass prisms. Draw a ray diagram to show the path of light when two identical glass prisms are arranged together in inverted position with respect to each other and a narrow beam of white light is allowed to fall
- White light is a mixture of lights of seven colours, red, orange, yellow, green, blue, indigo and violet. The dispersion of white light occurs because colours of white light travel at different speeds through the glass prism. The amount of refraction depends on the speed of coloured light in glass.
- When white light consisting of seven colours falls on a glass prims, each colour in it is refracted by a different angle, with the result that seven colours are spread out to form a spectrum. The red light bends the least, while violet light bends the most.
Newton’s experiment with two identical prism:
- When a beam of white light is passed through a glass prism, a band of seven colours is formed on a white screen. This band of seven colours is called spectrum of white light.
- Newton showed that the seven coloured light of the spectrum can be recombined to give back white light.
- First he tried to split the colours of the spectrum of white light using a prism.
- He then placed a second identical prism in an inverted position with respect to the first prism. This allowed all the colours of the spectrum to pass through the second prism. He found a beam of white light emerging from the other side of the second prism.