Emission and absorption spectra

Emission spectra

You should have a look at various spectra (some of which are shown in the diagrams). You will see that they fall into roughly three groups.

(i) line spectra - these are emitted from hot monatomic gases - the atoms are not linked to each other in any way


(ii) band spectra - these are emitted by gases as well but from ones with more than one atom per molecule


(iii) continuous spectra - these are emitted from hot solids - the intensity /wavelength distribution depending on the temperature of the solid

Absorption spectrum

This type of spectrum is formed by the passage of light through a cooler vapour. The vapour then absorbs those regions of the spectrum that it would have emitted had it been in an excited state. In the cooler gas most of the atoms are in the ground state.
Absorption occurs by the electrons in the atoms absorbing energy from the incoming radiation and then reradiating it in all directions.


In 1814 Fraunhofer discovered that the spectrum of the Sun's radiation was crossed by hundreds of absorption lines that he called Fraunhofer lines. These are due to the absorption of light from the centre of the Sun by the outer and cooler layers. Study of these lines gives the astronomers a way of determining the composition of the Sun.