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What is an Impure Spectrum? What is Pure Spectrum?
If it was possible to isolate a single ray of white light and pass it through a glass prism it would split up into seven distinct and separate single colours producing what could be called a pure spectrum. In actual practice, It is not possible to isolate a single ray. Even If a narrow pencil is taken, it will contain a large number of rays and if the pencil is divergent, each ray in the pencil passing through a prism will produce a spectrum of its own on the screen and these different spectra will partially overlap. No colour can be separately identified. As a result the spectrum becomes impure, as illustrated in the Fig.
The spectra produced by the rays between SA and SB lie in the region R1V2. So the colours overlap each other producing impure spectrum.
Definition: Impure spectrum is a spectrum in which the constituent colours overlap each other and hence cannot be distinctly separated from one another.
Pure spectrum is a spectrum in which all the constituent colours occupy different and distinct positions and do not overlap one another.
Method of Producing Pure Spectrum
A source of white light illuminates the narrow vertical slit S [Fig.]. The slit is placed at the principal focus of a convergent lens L1 which renders the emergent rays parallel. The emergent rays are received by the prism P placed in the position of mini-mum deviation for yellow rays. The dispersed rays come out of the prism and proceed in parallel directions. A second convergent lens L2 placed beyond the prism brings the different groups of parallel rays to different foci, violet at V and red at R on the screen M, the foci of other colours lying in between them. Thus a real pure spectrum is formed on the screen M which is placed at the focal plane of the lens L2.
Conditions for formation of pure spectrum: For producing pure spectrum the following conditions should be satisfied:
- The slit should be narrow. Otherwise many light-rays being incident on the prism will produce impure spectrum.
- A convex lens should be placed between the slit and the prism, so that the slit is at the principal focus of the lens and a parallel emergent beam may fall on the prism.
- The prism should be placed in the position of minimum deviation for the mean rays (i.e. yellow light) so that all other rays emerge with minimum deviation.
- The refracting edge of the prism should be parallel to the length of the slit.
- A convex lens with a suitable focal length should be placed beyond the prism so as to converge all the parallel rays of identical colour emerging from the prism to a single point on the screen.
- The screen M is to be placed at the focal plane of the lens L2.
- All the lenses used must be achromatic.