General Physics Question:

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