Shells

sweep the light. the atom takes only a few.

drag a needle along a spectrum to shine light on a hydrogen atom: only photons whose energy matches a gap between levels are absorbed, the electron jumps and falls back printing that spectral line; find the Balmer lines, then the ultraviolet Lyman lines, then free the electron

A hydrogen electron can only have the energies -13.6 eV / n². A photon is taken only if its energy is exactly the gap to a higher level; every other colour passes straight through. The electron then falls back and gives out the same energy, which is why a gas shines in lines: Balmer (from n=2, the four you can see), Lyman (from n=1, ultraviolet), and past 91.2 nm the ladder ends and any photon frees it. Here the Balmer round starts on n=2, as many atoms do in a hot star, and each fall goes straight back; the last cascade is one rung at a time, one of several real paths. The ellipses are the Bohr-Sommerfeld model of 1916, drawn as a model and not to scale: the energies are real, the orbits are not.

Concepts: quantum-mechanics, atomic-structure.

An interactive you play with one thumb, on Wundr.