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Q & A: Why do fluorescent things glow when exposed to Ultraviolet light?

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Most recent answer: 04/14/2015
Why do florescent things glow when exposed to Ultraviolet light?
- Will Moyes (age 14)
Oak Harbor, WA


Atoms are made of electrons which "orbit" around a nucleus. The electrons can be in different states, and which state they exist in depends on how much energy they have. If an electron gains or losses energy, it changes its state.

Light is a form of energy. When the energy is absorbed by atoms or molecules, that mostly happens by single lumps of energy, called "quanta".  The frequency of light tells you how much energy each quantum has, with higher frequencies having more energy. The frequency of light also tells you what color it is; red light has a lower frequency than blue light does. Your eye is only able to see frequencies which are between red and blue. Ultraviolet light has a frequency which is higher than blue light, and is too high for your eye to see.

Some materials have a property that the electrons in their atoms are able to absorb the energy from ultraviolet light and change their state to one which has more energy. The electron doesn't stay in the higher-energy state forever, though. The electron eventually falls back into the lower energy state, releasing the extra energy into light again. But the fall isn't perfectly efficient; some of the original energy from the ultraviolet light gets lost to other things, for example into making the atom hotter. The quantum of light which is released doesn't have as much energy as the ultraviolet light did. It now has a lower frequency, which your eye IS able to see. So it looks like the object is glowing.

-Courtney K

(published on 04/14/2015)

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