atomic spectra
Atomic spectra are the specific set of colours that each kind of atom emits or absorbs, as unique to each element as a barcode or a fingerprint. Everyday image: pass the light from a glowing gas through a prism and, instead of a smooth rainbow, you see a few sharp bright lines at particular colours. Hydrogen has one pattern, neon another, sodium another. The question they answer: why does a hot gas glow only in certain pure colours rather than every colour at once?
Precisely, an atom emits light only when one of its electrons drops from a higher energy level to a lower one, releasing a photon whose energy equals the gap: E_photon = h f = E_high minus E_low. Because the energy levels are quantized (they come in a fixed set), only certain gaps, and therefore only certain colours, are possible. An emission spectrum is the set of bright lines a hot gas gives off; an absorption spectrum is the matching set of dark lines a cool gas removes from white light passing through it. For hydrogen the visible lines follow a simple pattern captured by the Rydberg formula.
Why it matters: atomic spectra are how we know what distant stars and nebulae are made of, without ever touching them, simply by reading the fingerprint lines in their light. Spectra confirmed the Bohr model and the whole idea of energy levels, and today they underpin lasers, neon signs, and chemical analysis. Honest note: real spectra also show fine splittings, widths, and shifts that the simple Bohr picture cannot fully explain, and which require full quantum mechanics.
A sodium street lamp glows a distinctive yellow because sodium atoms emit two closely spaced yellow lines near 589 nanometres. Seeing those exact lines in the spectrum of a distant object is proof that it contains sodium.
Each element's spectrum is a unique fingerprint of light.
Emission gives bright lines and absorption gives dark lines at the very same colours, because both come from the same fixed set of energy-level gaps.