Question: what makes something glow in the dark?

Susan Cartwright answered on 19 Jun 2015:

A variety of things.

Some radioactive elements, e.g. radium, are active enough that they glow in the dark. Luminous hands on watches used to be coated with radium. They stopped doing that when they realised that the workers who made the watches were developing radiation-induced cancers.

Many deep-sea animals, and some insects such as glow-worms and fireflies, produce light by chemical reactions (bioluminescence). The reaction is between a protein (luciferin) and an enzyme (luciferase): it is an exothermic reaction, and part of the energy release is given off in the form of light.

Some materials are phosphorescent: they absorb light in the daytime, store the energy in the form of electrons excited to a higher energy state, and subsequently emit light when the electrons go back down to their ground state. Most materials that can do this are fluorescent: the light is emitted immediately, e.g. in fluorescent lighting (this can still seem to be “glow in the dark”, because sometimes these materials absorb in the ultraviolet but emit in the visible, so invisible “black lights” can cause the glow). However, in some cases the excited state has a very long lifetime, because it is quantum mechanically difficult for it to emit a photon. These are the phosphorescent materials, and they are the basis for most “glow in the dark” paints and stuff.