Atomic spectra emission and absorption: WACE Year 12 Physics Unit 4

What this dot point is asking

WACE wants you to connect quantised energy levels to the observed line spectra, distinguish emission from absorption, and explain why spectra identify elements. This is the experimental evidence that energy levels are real and discrete.

Why spectra are lines, not bands

If atomic energy could take any value, atoms would emit a continuous range of wavelengths. Instead, electrons can only sit on specific levels, so the allowed transitions, and therefore the emitted or absorbed photon energies, are limited to a discrete set. Each transition gives one sharp frequency, seen as a single line. The pattern of lines maps directly onto the pattern of energy-level differences.

Emission spectra

When atoms are excited, by heating or an electric discharge, electrons jump to higher levels and then fall back, emitting photons of definite energies. Viewed through a spectrometer, this appears as bright coloured lines on a dark background. A neon sign and a sodium street lamp show their characteristic colours for exactly this reason.

Absorption spectra

If white light, containing all visible wavelengths, passes through a cooler gas, atoms absorb photons whose energies match their level differences, lifting electrons to higher levels. Those exact wavelengths are removed from the transmitted light, leaving dark lines in an otherwise continuous spectrum. The absorbed and re-emitted light scatters in all directions, so it is missing from the forward beam.

A fingerprint for each element

The energy levels depend on the number of protons and the arrangement of electrons, which is unique to each element. So the set of spectral lines, their positions and spacings, is a unique signature. The dark Fraunhofer lines in sunlight reveal which elements are present in the Sun's outer layers, and the same method identifies the composition of distant stars.

Counting the possible lines

A common question gives a set of energy levels and asks how many spectral lines are possible. Each distinct pair of levels gives one transition, so $n$ levels produce $\dfrac{n(n-1)}{2}$ lines: three levels give three lines, four levels give six, five levels give ten. The largest energy gap (from the top level to the ground state) gives the highest-energy, shortest-wavelength line, while the smallest gap between adjacent levels gives the lowest-energy, longest-wavelength line. Sketching the levels and drawing every downward arrow is the reliable way to make sure you count each transition exactly once and can then rank the lines by wavelength.

Using spectra to study stars

The practical power of this topic is that spectra let us learn the composition, temperature and motion of objects we can never sample directly. The dark absorption lines in sunlight (the Fraunhofer lines) reveal which elements are present in the Sun's cooler outer layers, because each set of missing wavelengths matches a known element's fingerprint. The same method identifies the elements in distant stars and galaxies. Beyond composition, the lines also shift: if a star is moving away, all its lines are red-shifted to longer wavelengths, and the size of the shift gives the speed of recession. This is the link to cosmology, where the systematic red-shift of distant galaxies is evidence for an expanding universe. So a single spectrum carries a remarkable amount of information once you can read the line positions.

Emission versus absorption in answers

State the direction of the electron jump: emission lines come from downward transitions (bright lines), absorption lines from upward transitions (dark lines against a continuous spectrum). The line positions are identical for both because they reflect the same level differences.