CHEMISTRY - Concept of Emission and Absorption Spectra

An instrument used to separate the radiations of different wavelengths is called Spectroscope or a spectrograph. A spectrograph consist of a prism or a diffraction grating for the dispersion of radiations and a telescope to examine the emergent radiations with the human eye. However, if in a spectroscope , the telescope is replaced by a photographic film, the instrument is called a spectrograph and the photograph of the emergent radiation recorded on the film is called a spectrogram or simply a spectrum of the given radiation. The branch of science dealing with the study of spectra is called spectroscopy.
The spectra are broadly classified into :-
1. Emission Spectra.
2. Absorption Spectra.

1. Emission Spectra :- When the radiation emitted from the some sources, e.g. from the sun or by passing electric discharge through a gas at low pressure or by heating some substances to high temperature etc. is passed directly through the prism and then received on the photographic plate, the spectrum obtained is called 'Emission spectrum'.

Depending upon the source of radiation, the emission spectra are mainly of two types :-

(A) Continuous spectra :- When white light from the any source such as sun, a bulb or any hot glowing body is analysed by passing through a prism , it is observed that it splits up into seven different wide bands of colours from violent to red, ( like rainbow ). These colours are so continuous that each of them merges into the next. Hence the spectrum is called continuous spectrum.
It may be noted that on passing through the prism , red colour with the longest wavelength is deviated least while violet colour with shortest wavelength is deviated the most.

(B) Line Spectra :- When some volatile salt is placed in the Bunsen flame or an electric discharge is passed through a gas at low pressure, light is emitted. The colour of light emitted depends upon the nature of substance. For example , sodium or its salts emit yellow light while potassium or it's gives out violet light. If this light is resolved in a spectroscope, it is found that no continuous spectrum is obtained but some isolated coloured lines are obtained on the photographic plate separated from each other by dark spaces. This spectrum is called 'Line emission spectrum' or simply Line spectrum.
Each line in the spectrum corresponds to a particular wavelength. Further, it is observed that each element gives its own characteristic spectrum, differing from those of all other elements. For example, Sodium always gives two yellow lines. Hence, the spectra of the element are described as their finger prints differing from each other like the finger prints of the human beings.

2. Absorption Spectra :- When white light from any source is first passed through the solution or vapours of a chemical substance and then analysed by the spectroscope, it is observed that some dark lines are obtained in the otherwise continuous spectrum. These dark lines are supposed to result from the fact that when white light ( Containing radiations of many wavelengths ) is passed through the chemical substance , radiations of certain wavelengths are absorbed, depending upon the nature of the element. This shows that the wavelengths absorbed were same as were emitted in the emission spectra. The spectrum thus obtained is , therefore , called 'absorption spectrum' .