Definition for Phosphorescence
From Biology Forums Dictionary
Phosphorescence is the ability of a mineral to glow after the initial activating ultraviolet light is removed. The electrons in the mineral have essentially stored the energy of the initial activating light and then re-emit the phosphorescent light on a delayed basis. The reason for the delay is because a certain amount of the electrons are prevented from returning to their lower energy states quickly, at least not as quickly as they were initially energized by the UV light. Most children are familiar with "glow-in-the-dark" toys, t-shirts and stickers that make use of phosphorescent chemicals. It is quite a unique thing to see this occur in a natural mineral. Some phosphorescent intensity is very low and actually beyond the capabilities of the human eye to detect. But specimens that have been exposed to UV light and then placed in front of sensitive photographic film can be reveal to be phosphorescent for some time after the initial activation, by producing an exposure on the film. In some extreme cases for years afterward. This is not a consistent property to observe and is not very diagnostic for this reason.