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I've noticed that whenever I turn the lamp off in my room at night, the lightbulb seems to continue to glow for a minute or so after that. It's not bright though; the only way I even notice it is if the room is dark. So why does it keep glowing?

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surely there has to be a better tag than optics for this question, ideas? –  Sklivvz Nov 17 '10 at 20:40
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@Sklivvz Condensed-matter, blackbody-radiation, household-physics, elecricity, heat, energy. –  Mark C Nov 17 '10 at 23:56
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3 Answers 3

up vote 14 down vote accepted

If it's an incandescent bulb, it's because the whole operating principle of the bulb is based on getting the filament really hot, hot enough to glow. When you cut off the current, it stops heating the filament, so it cools down fairly rapidly, but there may be enough residual heat for a faint glow lasting a little while afterwards.

If it's a CFL bulb or an ordinary fluorescent bulb, the "white" color we see is produced by a fluorescent coating on the glass of the bulb, which converts some of the invisible ultraviolet light produced by the excited gas atoms into visible light. When you cut off the current, you stop exciting the gas atoms and thus stop producing new ultraviolet light, but there may be enough residual excitation in the material to keep glowing for a little while afterwards. This would be similar to the way glow-in-the-dark materials work-- those absorb UV light when they're in bright light, exciting atoms and molecules in the material, which then slowly emit visible photons.

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It is still hot enough to emit light.

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Insufficient data! –  Mark C Nov 17 '10 at 23:56
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The filament will cool very rapidly after current stops flowing and I would say will only account for a second or two of your perceived after glow. If you are noticing a longer duration of glow (you state a minute or so) that is probably retinal in cause retinal afterimage

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