Pure spectrum : When rays of white light pass through a prism, each ray forms a spectrum on the screen. If each colour is seen separately in the spectrum, then this type of spectrum is called 'pure spectrum'. The following are the conditions for obtaining a pure spectrum :
(i) The line aperture should be narrow and located at the focus of the converging lens.
(ii) The prism should be in a position of minimum deviation.
(iii) Rays of the same colour emanating from the prism should be focused at one place by the converging lens.
(iv) The incident rays should be parallel.
Impure spectrum : When rays of white light pass through a prism then each ray produces a spectrum on the screen. Due to overlapping of different colors, these colours are not visible separately on the screen. This type of spectrum is called 'impure spectrum'.