Absorption spectrum

A related phenomenum to the emission spectra of elements is the absorption spectra. Imagine that we have an electron in a lower energy state, and a photon comes along. If this photon has just the right amount of energy, it can be absorbed, causing the electron to make a transition to a higher energy state. This is illustrated in the figure below.

 

Figure 12.6: Photon absorption

This will only occur though if the photon has an energy equal to the energy difference between the two levels involved in the transition - if not, the photon will just pass through.

Such an effect can be seen by shining light of all different frequencies through a gas of a particular element. Some of the photons will be absorbed in the gas, and so will be missing in the light that emerges from the gas. What one will therefore see from the emerging light is an almost continuous band of frequencies, but with some frequencies missing corresponding to the absorbed photons.

As with the emission spectrum, each element has its own unique absorption spectrum, as the energy levels of different elements differ. Measuring the absorption spectrum can therefore be used to identify elements in an unknown substance by comparing it to the spectrum of known elements. Among other areas, this is a major technique used to identify elements in gaseous clouds in galaxies.