Spectra & Atomic Energy Levels
Emission Spectra
- When an electron transitions from a higher energy level to a lower energy level, this results in the emission of a photon
- Each transition corresponds to a specific wavelength of light which correlates to an observable spectral line
- The resulting emission spectrum contains a set of discrete wavelengths, represented by coloured lines on a black background
- Each emitted photon has a wavelength which is associated with a discrete change in energy
Emission spectrum of hydrogen gas
Absorption Spectra
- An atom can be raised to an excited state by the absorption of a photon
- When white light passes through one side of a cool, low-pressure gas it is found that certain wavelengths of light are missing when detected out the other side
- This type of spectrum is called an absorption spectrum
- An absorption spectrum consists of a continuous spectrum of all wavelengths (i.e. white light) with dark lines where specific wavelengths have been absorbed by the gas
- These dark lines correspond exactly to the differences in energy levels in an atom
- When electrons return to lower energy levels, the photons are emitted in all directions, rather than in the original direction of the white light
- Therefore, some wavelengths appear to be missing
- The wavelengths missing from an absorption spectrum correspond to the coloured lines on an emission spectrum for the same element
Absorption spectrum of hydrogen gas