So basically in my toilet I have this really bright ceiling light, but when I turn it off it doesn't just instantly plunge me into darkness, but rather sort of fades out over a few seconds. It looks just like a bulb light to me, but it's pretty high up so I can't check it.

Conjecture: the bulb works by heating a wire, which at high temperatures gives off light, and when I flick off the switch it doesn't just instantly cool down, it takes a while, and with residual heat gives off less and less light with each passing second. But if this is the case, I'm still a tad confused. I thought that for radiation like this the wavelength should vary with the heat produced? If so, when it dims, shouldn't the light change color to a more reddish and pinkish (less energetic) wavelengths? Instead what I observe is that it just keeps the same warm yellow glow, just getting less and less bright as time goes on. Or are my eyes too bad at detecting such a small change?

Tried searching on google and physics stackexchange (seems like a pretty common problem) but for some reason can't find it anywhere. Maybe I'm wording it badly.

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    $\begingroup$ "Warm yellow glow" seems to imply a low-pressure sodium lamp, which are indeed monochromatic, but they are rarely used indoors, maybe only for emergency lighting in older buildings. Although you can't get to the bulb, you can still use a prism to check the composition of the light, which can be used to figure out what kind of device it's coming from. $\endgroup$ – biziclop Oct 28 '14 at 11:11
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    $\begingroup$ @biziclop a warm yellow glow can be obtained with LED or halogen lamps, both commonly used indoors. Good point in measuring, though. $\endgroup$ – Davidmh Oct 28 '14 at 11:47

You said that the lamp gave off a yellow glow, so it is possible that it could be a sodium lamp.

However, your conception about light intensity and wavelength is a bit off. If the lamp that you are speaking of gives off a monochromatic light source, it is most likely using an electrical current to excite the atoms of a single element. When excited, different atoms only emit certain wavelengths of light that are visible to the human eye. Here is sodium, which only emits yellow light.

As for the fading intensity, it is possible that it happens the way that you say, however, that would mean that it would take a bit of time for the light to shine at it's full capacity (which is actually common as far as sodium lamps go). Sodium also takes a while to heat, so it is common to include neon (which is excited at a lower temperature, and emits a red wavelength), so that the light can glow until the sodium begins to vaporize.

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  • $\begingroup$ If it is black-body radiation though (and incandescent light bulbs are approximately that), then the wavelengths do indeed shift towards the red. (Or rather, the overall perceived colour.) $\endgroup$ – biziclop Oct 28 '14 at 11:04

You speak of fading over seconds. This is not likely to be a result of filament glow in a mains voltage domestic light as they cool very fast.

There is unlikely to a sodium vapour lamp in your bathroom either.

Most LED lights will turn on and off pretty fast as storage capacitors are expensive to waste.

Possibly a thick filament (low voltage) halogen bulb could cause the glow for that length of time.

The colour cast you are expecting will be very hard to perceive with any lamp technology. The human visual system does automatic white balance correction continuously and within limits saturated colours look much the same in candle light or under a cool white florescent light.

The long duration effect you describe I have observed with phosphor based illumination, most pronounced with a CRT display or TV and also to a lesser extent with certain fluorescent lights. The mineral mixes that are used in many lamp coatings also have some phosphorescence resulting in afterglow. Depending on the specific lamp this may be unnoticeable or more pronounced.

You can demonstrate this simply with a bright torch and a fluorescent tube (no electrical connection required). In total darkness allow your eyes to accommodate somewhat to perceive weak effects. Now align torch with one end of tube, close and cover eyes and shine light on tube for 5 seconds. Turn off torch and open eyes and you will see part of tube glowing from the phosphorescence (light storage), no fluorescence (UV excitation) took place. Because the effect is slower it appears that the light is still glowing after the power has been turned off.


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    $\begingroup$ Yes, I've noted that cooldown of 2 or 3 seconds in 12V halogen lamps. I've observed it in 240V halogens too, but there it will last for about half a second or even a bit less. $\endgroup$ – rodrigo Oct 28 '14 at 10:49
  • $\begingroup$ @rodrigo Projector lamps also have even longer glow because of the thick and compact filament, even though they may be high voltage (80V in the past), required to get the light to appear from one spot. Some HID lamps may have afterglow from an incandescent electrode. $\endgroup$ – KalleMP Oct 28 '14 at 11:20

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