Three main ways. Radioactive, light absorbing material and chemical reaction.
Radioactive: In the good old days say before the 80's or 70's they used Radium. They painted watch & clock dials and instruments with radium paint which by its name you can tell is radioactive. It glowed and was radioactive. It was such a low back ground level, especially when in a case, not on your skin, nose or mouth, it was not a health problem. However exposing it direct to skin or disposing of it can be a problem. Radium is not allowed anymore. New watches still may use a radioactive substance (not radium) but a more stable isotope. The down side is they are not as bright or last as long.
GLOW IN THE DARK MATERIALS
The common kind of "GLOW IN DARK" like in toys you see today is from materials that will absorb UV light, than glow in low light for a limited time. A common substance is strontium aluminate phosphorescent pigments. Exposing the material to fluorescent light or ultraviolet light energizes the phosphor and will bring it to a higher engergy level (on an atomic level). It is like charging a battery. It gives off the energy as a light until it gets back to its normal state. Think of it like a hot pot that cools. You "heat" the material up but not with tempature but light. It glows as it "Cools".
PURE CHEMICAL REACTION
There are pure chemical reactions like glow sticks requres a catalyst to start the reaction called chemoluminescence. Electrical power or absorbing external light or radioactive material is not needed. The glow stick contains two chemicals and a suitable fluorescent dye (sensitizer, or fluorophor). The chemical in the plastic tube is a mixture of the dye and Cyalume. The chemicals are hydrogen peroxide and phenyl oxalate ester. A glass tube breaks to mix the chemicals and start the reaction. It is bright and will last 6-12 hours.
These are the big three I know of.
· 1 decade ago