Spectra and its kind: the colored pattern obtained on a screen after dispersion of chromatic light is known as spectrum. A spectrum without overlapping of colors is called pure spectrum and a spectrums with colors overlapped is called impure spectrum. To obtain pure spectrum:
I. The source of light should be used in the form of narrow slit.
II. Light should be collimated as beam of parallel rays.
III. Prism should be kept in minimum deviation position.
IV. Emergent rays are focused by another convex lens.
Spectrum is a device for analyzing a beam of lights into its constituents colours. A spectrometer consists of a collimator, a prism table and a telescope. There are two kinds of spectra:
I. Emission spectra and
II. Absorption spectra.
The spectrum for the radiation emitted by a luminous body which is obtained by passing the light of the luminous body through a prism (or grating) is known as spectrum. Emission spectra is bright spectrum on dark background. It is characteristic of source of light.
The spectrum obtained on screen by passing a light of a source and then by passing the transmitted light through a prism (or grating) is called absorption spectrum. The spectrum is the absorption spectrum of that substance as the substance in unexcited state absorbs few radiations emitted from the light source. Certain wavelengths are absent in absorption spectra which appears black in the spectrum. Hence, absorption spectra consist of few dark lines. The fraunhofer lines are the dark lines in absorption spectra of the radiations emitted from the sun as the light emitted from core of the sun (photosphere), passes through sun’s atmosphere (chromospheres), few wavelengths are absorbed by gas present there, resulting the dark lines. These emission spectra and absorption spectra are each divided into three categories: Continuous spectrum, line spectrum and band spectrum.
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