Dispension:-Light consists of seven columns. They are

violet, indigo, blue, green, yellow, orange and red. This

coloured bend i.e. called spectrum and the phenomenon of

white light splitting up into seven constituent columns is

called dispersion

Spectrum:-To get a pure spectrum light from a slit is

allowed to pass through a convex lens placed in such a

position that a clear image of the slit is formed on the

screen. A spectrum will appear on the screen on inserting a

prism between the screen and the lens. On rotating the

prism, the spectrum will move towards the original image and

after sometime will become stationary

The essentials for pure spectrum are:-

1.The slit must be very narrow.

2.The prism should be paced in the position of minimum deviation

3.The rays of light must be paralled to each other as they

fall on the prism

4.A convex lens must be used to focus rays of different

column on the screen