Emission and Absorption Spectra

Spectra in general can be classified into two major groups, viz. emission spectra and absorption spectra. Emission spectra are due to the light emitted from (i) incandescent solid or vapour (ii) an arc discharge, (iii) a spark discharge, (iv) a sodium vapour lamp, (v) a mercury vapour lamp, (vi) the discharge of electricity through a gas or vapour contained in a discharge tube etc.

When a beam of light is incident on a transparent medium, a part of the light is reflected and the rest is transmitted. But some material media behave differently. Certain materials absorb strongly light corresponding to a particular region of the spectrum and transmit the rest. This is termed as selective absorption. The resulting spectrum of the transmitted light consists of dark lines or bands called the absorption bands. However, if the light is incident on lamp black or platinum black the whole of the incident light energy is absorbed and there will be no transmitted light. This is called general absorption.

If a beam of white light is passed through cool sodium vapour, light corresponding to the wavelengths of D₁ and D₂ lines is absorbed and the rest is transmitted. Thus, the spectrum of the transmitted light consists of all the wavelengths except those corresponding to the absorbed wavelengths. Similarly, a piece of glass coloured with copper oxide (ruby glass) transmits red light and absorbs all the rest of the wavelengths.

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