In this topic of IBDP Physics, we will learn about Line Emission and Absorption Spectra.
Line Emission Spectra
As the energy levels have different values, each of the possible electron transitions within an atom will produce a photon with a different energy. This means that each electron transition will produce a photon of different frequency and hence a different colour.
In IBDP Physics, this causes line emission spectra to be produced. Because different types of atoms have different energy levels, the photons produced will be different and so the line spectra for different elements will be different. Line spectra can be used to identify elements.
These spectral lines are the result of many millions of energy level changes – which are all the same, since each atom of an element is identical in structure, and there are many millions of atoms in even the tiniest sample of element.
In IBDP Physics, the brightness of a spectral line is determined by the number of electrons making the particular energy change which causes it:
Line Absorption Spectra
- Sodium flames absorbs photons to fill energy gaps
- Emits photons in all directions
- Black lines appear where the colours of these particular energies would have been
Before the sodium flame is lit, the white light source produces a full spectrum of photons resulting in a complete colour spectrum being visible at the telescope. Once the sodium flame is lit, black lines or ‘missing colours’ appear in the spectrum.
In IBDP Physics, the photons within the colour spectrum which have the same energies as the photons emitted by the sodium vapour, have exactly the right amount of energy to cause the electrons in the sodium atoms to ‘jump up’. The energy from these passing photons is absorbed as it is used to promote the electrons in the sodium atoms.
The promoted electrons drop back down almost immediately, and in doing so emit photons associated with their particular change in energy levels. These photons have identical frequencies, and hence colours, as the photons which were absorbed initially.
However, the emitted photons are not emitted in the same direction as those that were absorbed. The photons are emitted in all possible directions, with only a few travelling towards the collimator and the telescope. Thus the intensity of the colours due to those particular photons is very low – resulting in black lines where the colours would have been.
This is the end of this topic.
