By Sunil Bhardwaj

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Monochromator are the dispersing devices used in spectroscopy, Monochromator produces light of single wavelength.

Prisms: The collimated beam is dispersed by the prism into its constituent wavelengths. As the prism is having different thickness, light with different wavelengths gets diffracted with different angles and get separated. The prism is rotated to allow a particular wavelength to fall on a focusing lens and then emerge out of an exit slit.

Diffraction Gratings: The grating Monochromator have some advantages over prism which are as follows:
1. A grating gives much better dispersion of light than prism.
2. As the gratings are usually made up of non-corrosive material like aluminium which is not easily attacked by moisture. But metal salts prism are attacked by atmospheric air easily.
3. Gratings can be used for over a larger range of wavelength as compared to prism.

A diffraction grating is made by ruling a large no. of parallel & equidistant lines on highly polished surface of a metal such as Al. For UV and Visible lights these lines must be 15000 - 30000 lines per inch. But for I.R. it should be 1500-2500 per inch.

The light which strikes the ruling is scattered and dissipated (wasted). But the light which strikes the unruled portions or grooves is reflected regularly. The reflected waves produce interference pattern which results in the dispersion of light into its component wavelength. The above type of the gratings is known as Reflection Grating.

The other type of grating is known as Transmission Gratings which are obtained by ruling large no. of parallel and equidistant lines on glass. When the light falls on transmission grating each groove acts as an individual source of light. The transmitted rays from these grooves produce interference pattern which results in the dispersion of the transmitted light into its components wavelength.

The conditions for constructive interference are given by $$n\lambda = d\left( \sin { i } + \sin { r } \right)$$ where $$n = + ve$$ integer,
$$\lambda$$ = wave length of light incident,
$$d$$ = distance between two grooves. (It is known as grating constant)
$$i$$ = angle of incidence
$$r$$ = angle of refraction

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