Fraunhofer lines are the black lines on an emission spectrum of an element. The spectrum of sunlight has Fraunhofer lines in it which shows that some wavelengths were missing from the continuous spectrum. The lines are caused by the absorption of some colours rather than emission. This is why:
- The interior of the sun produces white light, with all wavelengths present
- As this light passes through the sun's atmosphere, some wavelengths are absorbed by the atoms present
- So the sun's light that reaches us is missing some wavelengths, which correspond to the elements in the sun's atmosphere.
The line spectrum of an element was used to discover helium. A scientist noticed a line in the spectrum that he hadn't seen before. He sent his observation to another scientists who realised that the line did not correspond to any known element. He guessed that another element was present and named it, "helium".
How atoms emit light
Basically: when an atom loses energy, the energy they lose is carried away by light
- The electrons in an atom can only have certain values of energy. Scientists think of them as occupying points on a 'ladder' of energy levels.
- When an electron drops from one energy level to another, it loses energy.
- As it does so, it emits a single photon of light- a packet of energy. The energy of the photon is equal to the difference between the two energy levels.
The greater the energy gap, the higher the energy of the photon
High energy photon = high frequency, short wavelength light
Tip: Think of the ladder of energy levels as a staircase which lots of people are sitting on. Some stairs aren't available because they're too full, so the people (photons) have to jump to the ones available.
Where Fraunhofer lines come from
The 'ladder of energy levels' model can be used to explain the absorption spectrum with the Fraunhofer lines.
- White light consists of photons with all possible values of energy
- An electron in a low energy level can only absorb a photon whose energy is just right to lift it up to a higher energy level.
- The white light is now missing photons that have been absorbed because their energies corresponded to the spacing in the ladder of energy levels. The 'missing' photons correspond to the dark lines in an absorption spectrum