Emission Spectra

About this topic

When atoms are heated or electrically excited, electrons jump to higher energy levels, then fall back and emit photons with energies equal to the difference between levels. Each element therefore produces a unique pattern of coloured lines, its line spectrum, like a fingerprint.

The energy of a photon is E = hf, where h is Planck's constant and f is the frequency, so bigger jumps give higher-energy, bluer light. Because electron energies are quantised, only certain jumps are possible, which is why we see sharp lines instead of a smooth rainbow. Hydrogen's visible lines include red, blue-green and violet, produced when electrons fall to the second level. An absorption spectrum shows dark lines at the same positions when white light passes through a cool gas.

Picture a ball on a staircase that can only rest on certain steps: when it drops from a high step to a lower one, it gives out a fixed amount of energy. Flame tests use this idea: sodium glows yellow, copper blue-green, potassium lilac and strontium red, which is why fireworks have their colours.

Astronomers identify elements in distant stars from their spectra, and helium was first found in the Sun this way. A misconception is that atoms emit light when electrons move up; they absorb energy going up and emit it coming down.