atomic spectra

When you pass the light from a glowing gas through a prism, you do not get a continuous rainbow but a set of sharp, separate bright lines at very specific colours. Pass white light through a cool gas instead and you see the same colours missing, as dark lines. Each chemical element has its own unique pattern of lines, a kind of optical fingerprint that lets astronomers read the composition of distant stars from their light alone.

This discreteness was a profound mystery for classical physics. If an atom were a tiny solar system of electrons orbiting a nucleus, the orbiting electrons should, by Maxwell's electromagnetism, radiate energy continuously, spiral inward, and crash into the nucleus in a fraction of a second — emitting a smear of all colours on the way down. Atoms manifestly do neither: they are stable, and they emit only certain sharp colours.

The lines also obeyed strikingly simple numerical patterns, begging for an explanation. The resolution, when it came, was revolutionary: an atom can hold only certain discrete amounts of energy, and a spectral line is emitted when an electron jumps between two of these allowed levels, releasing a photon whose energy is exactly the gap between them. Atomic spectra are thus a direct, visible readout of energy quantization inside matter.