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Q & A: Glow in the dark

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Q:
Why and how do objects glow in the dark?
- Erin Miller (age 12)
Treasure Mountain Middle School, Park City, Utah, USA
A:
There are several different ways that things glow. Black-body radiation, the kind you get from a light bulb, is caused by heating wires in the bulb to such a high temperature that they glow, emitting light. Other kinds of light are called cold luminescence, because they don't use heat energy to create light. What happens in cold luminescence is the electrons of the atom are given some extra energy, and they become excited. This higher energy state is unstable, and so after a while the electron releases its extra energy in the form of a photon, a little packet of light. This is called fluorescence. Another interesting fact is that things that glow in the dark can do so in different colors (different wavelengths = different colors). Most glow-in-the-dark objects, like the plastic stars you can put on the ceiling of your bedroom, absorb energy from light (black-body radiation) and then the excited electrons slowly decay back down, emitting the glow you see. These materials also glow in the light, but the glow is so weak that you can't see it. Another way that objects can be made to glow in the dark is chemiluminescence, in which you combine two chemicals, and a byproduct of the reaction that occurs is energy that is given off in the form of light.

(republished on 07/28/06)

Follow-Up #1: glow-in-the -light

Q:
do glow-in-the-dark objects stop glowing when somebody turns the lights on? explain why or why not.
- dinna m. luna
san juan city, philippines
A:
They keep glowing in the light. Light doesn't turn off any of the processes that lead to the glowing. In fact, for the most common glow-in-the dark objects (phosphorescent), light is necessary to get the molecules into the high energy states which later emit light at a different color. So they glow even more in the light than in the dark, but it's just harder to see. When the light is turned off, they gradually glow less and less, as the energy they have stored in the light gradually gets lost.

Mike W.

(published on 09/04/07)

Follow-up on this answer.