Shells

fire energy at an atom. it accepts only exact amounts.

drag a photon along the energy axis and let go of it at the atom — wrong energy passes straight through, exact energy jumps

An electron bound to an atom can only sit at specific energies, never in between -- the ladder has no middle rungs. A photon only gets absorbed if its energy matches the exact gap between two of those levels; anything else passes through untouched, no matter how close it comes. Give the electron enough energy in one hit and it leaves the atom entirely, which is what ionisation is. This one fact is the reason every element absorbs and emits a fixed set of colours rather than a smear -- why neon signs, sodium streetlights and the dark lines in a star’s spectrum all carry a signature nobody drew, physics did.

Concepts: quantum-mechanics, atomic-structure.

An interactive you play with one thumb, on Wundr.