Terry Grant, of Levin, asks :-

My wristwatch had a luminous dial but with time it faded so much as to be unreadable. However when I was in the bush one night I had cause to shine my flashlight on the dial, and when I returned to the tent my watch dial was luminous again. It did not last for long though. What caused it to be re-charged?

Andy Edgar, a physicist at Victoria University of Wellington, responded.

There are two types of paint used for luminous dials. The older radio-luminescent paint relies on a tiny fraction of the atoms in the paint being replaced by radioactive ones. The particles emitted from the radioactive atoms smash into the paint atoms, and eject energetic electrons from them, leaving behind an atom with an electron deficit, known as a “hole”. When electrons return to fill the holes, a “recombination” process, the excess energy is released as light giving rise to the continuous weak glow visible in the dark. Over time, the paint becomes damaged by the radiation and darkens, reducing the light output.

The newer “persistence phosphor” paints do not rely on radioactivity, but require prior exposure to daylight, during which the ultraviolet content can eject electrons from the paint atoms. A small fraction of these electrons are trapped at deliberately introduced impurity atoms and do not immediately return to fill the holes. However, statistically, they will do so over time. The closer they are to the holes, the higher that probability is. Once the paint is in the dark, there is a residue of trapped electrons which progressively fill the holes, emitting recombination light in the process, but being depleted in number as time proceeds. The resulting glow gradually subsides.

In both cases a secondary effect related to trapping may occur. After a period in the dark, all the electrons closest to a hole will already have returned, leaving a residue of distant ones. Torchlight can give these electrons just enough energy to free them, but not enough to reach a hole, so they are promptly retrapped. On average the number of electrons close to a hole will increase, so there will be a one-off surge in the rate of hole filling and light emission, as reported.