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Spectra & Fluorescence

01Spectra & Fluorescence

Line spectra

This section covers discrete atomic line spectra and how they provide evidence for discrete electron energy levels.

Discrete wavelengths from atoms

If radiation produced by excited atoms is separated by wavelength, for example with a diffraction grating, it can appear as individual spectral lines with gaps between them rather than as an uninterrupted spread of wavelengths.

The pattern follows from the discrete energies available to atomic electrons. Each transition has a particular energy difference, and therefore corresponds to a photon with a particular frequency and wavelength.

ΔE=hf=hcλ\Delta E=hf=\frac{hc}{\lambda}

The link between line spectra and atomic energy levels is:

  1. Atomic electrons have only certain permitted energies.
  2. The differences between those permitted energies also take particular values.
  3. A transition with energy change ΔE\Delta E produces or absorbs a photon for which ΔE=hf\Delta E=hf.
  4. Only the corresponding photon wavelengths therefore appear as characteristic spectral lines.

Because different elements have different arrangements of energy levels, their spectral-line patterns differ. The positions of the lines can therefore be used to distinguish one element from another.

E1E2E3E4line spectrum

Different downward transitions have different energy changes, so they produce photons at different discrete wavelengths.

Energy difference Photon frequency Photon wavelength
Larger ΔE\Delta E Higher Shorter
Smaller ΔE\Delta E Lower Longer

Exam Tip: To explain why a line spectrum is evidence for discrete levels, give the complete chain: discrete electron energies → discrete energy differences → discrete photon energies → only particular wavelengths.

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