Explain the experiment conducted by Newton to show that white light contains seven colours.

Explanation

Light rays of different colours travel with same speed in vacuum and air. But in any other medium, they travel with the different speeds and bend through different angles, which lead to the dispersion of light.
Newton showed that the reverse of dispersion of light is also possible. He kept two identical prisms close to each other-one erect and the other in an inverted position as shown in the figure. The light gets dispersed when it passes through the first prism. The second prism receives all the seven coloured rays. from first prism and recombines them into the original white light. This obervation show that sunlight is made up of seven colours.

it is essential that the two prisms used should be identical and placed in inverted position with respect to each other because then, the second prism cancels the dispersion caused by the first prism so that white light can be obtained.