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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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4 Answers 4

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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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  • $\begingroup$ Most LED bulbs also include phosphors, so that could be a source of the glow. $\endgroup$
    – ThomasW
    Apr 6, 2015 at 2:41
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It is still hot enough to emit light.

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  • $\begingroup$ Insufficient data! $\endgroup$
    – Mark C
    Nov 17, 2010 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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If it is a compact fluorescent lamp, it will be the fluorescent powder (phosphor). Even though it is not a high persistence phosphor like in radar tubes that is designed to glow for a few seconds to keep the picture on the screen, it still has a bit of persistence and afterglow. All fluorescent lights can be seen glowing a bit in a dark room after they are switched off. Even the old TV picture tube screens would glow if exposed to UV from a black disco light. If you held your hand in front of it then took the light away you would still see a shadow of your hand. If you wave something shiny under a normal fluorescent light that flickers at 100Hz or 120Hz, you will see alternate bluish and yellow images: bluish from the mercury discharge, and yellow from the phosphor.

Some LED lamps go out slowly over a few seconds due to the charge in the capacitor in their power supply.

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