Line spectra are fingerprints of discrete energy levels

Energy Levels & Wave-Particle Duality — AQA A-Level Physics

Emission spectra

When excited atoms de-excite, they emit photons at specific frequencies corresponding to the energyThe capacity to do work. Measured in joules (J). gaps between levels.
Viewed through a diffraction gratingAn optical component with many equally spaced parallel slits that produces sharp interference maxima at specific angles., these appear as bright coloured lines on a dark background.
Each element has a unique set of energyThe capacity to do work. Measured in joules (J). levels, so each element has a unique emission spectrumA series of bright lines at specific wavelengths produced when excited atoms return to lower energyThe capacity to do work. Measured in joules (J). levels, emitting photons..

Absorption spectra

When white light passes through a cool gas, photons with energies matching the gas's energy gaps are absorbed.
This produces dark lines on a continuous coloured background — the absorption spectrumA continuous spectrum with dark lines at specific wavelengths where photons have been absorbed by atoms, exciting electrons to higher energy levels..
The dark lines appear at exactly the same frequencies as the bright lines in the emission spectrumA series of bright lines at specific wavelengths produced when excited atoms return to lower energy levels, emitting photons. of the same element.
The absorbed photons excite electrons to higher levels. These electrons then de-excite and re-emit photons in random directions, removing them from the original beam.

Line spectra (not continuous spectra) provide direct evidence that energy levels in atoms are discrete.
A continuous spectrum would imply electrons could have any energy — which they cannot.

Examiner Tips and Tricks

In an absorption spectrumA continuous spectrum with dark lines at specific wavelengths where photons have been absorbed by atoms, exciting electrons to higher energy levels., the dark lines match the bright lines in the emission spectrumA series of bright lines at specific wavelengths produced when excited atoms return to lower energy levels, emitting photons. of the same element.
If asked 'what evidence supports discrete energy levels?', line spectra is the answer.