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I have a glass, which emits green light after charging it with sunlight. I though this is fluorescence, and only photons with shorter wave length (than the green light has) can excite the electrons of this material and can be reemitted as green light. At least I remember on something like that from my studies. Now the interesting part that when I wash it with warm water, it starts to emit light too. How do you explain that?

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    $\begingroup$ It is phosphorescence. The excited state exists for some time after the initial excitation. Heating the plate causes the excited state to decay more quickly. $\endgroup$ – MaxW May 6 '18 at 17:24
  • $\begingroup$ @MaxW I think I can verify this. If it is true, then it should be depleted relative fast after applying warm water. Just a min. :-) $\endgroup$ – inf3rno May 6 '18 at 20:12
  • $\begingroup$ @MaxW Hmm, it will take more than that. It can glow for several hours, and I guess this would just cut that time to half or third. I'll do an experiment with it tomorrow. $\endgroup$ – inf3rno May 6 '18 at 20:20
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This phenomenon is thermoluminescence. The incoming photons excite electrons of the material to a higher level, a metastable state, which fall back to a lower state and emit photons. The metastable state may last for only nano- or picoseconds, in which case it is termed fluorescence. If the electrons need a bit of a "nudge" and hang around for a few seconds or hours, it is termed phosphorescence.

In the extreme case of some materials, the metastable state can last for millenia, which can be useful for dating the age of the item. For example, when pottery is fired, atoms in a metastable state relax. From then on, energy accumulates from cosmic rays and radioactive decay of nearby minerals such as thorium or potassium. By heating the ceramic and measuring the amount of light emitted, one can approximate the age of the pottery.

Ceramics need to be heated to a few hundred Celsius, but your glass works in just warm water. There are similar infrared detection cards with phosphors that are charged using visible or UV light, and that emit visible light when triggered by low-energy IR.

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