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As above... Surely it must go somewhere?

My child asked me this question and it was a difficult one to answer.

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Related: Where does the dark go when I turn on the light switch? –  Alfred Centauri Jul 12 '13 at 16:58
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I love the questions that children ask. If you're spraying water with a hose, where does the water go when you turn it off? If you're pitching baseballs, where do they go when you stop? –  Mike Dunlavey Jul 12 '13 at 17:52
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In the fridge, of course. Don't believe me? Turn off your lights and open your fridge! :) –  TildalWave Jul 13 '13 at 1:33
    
Light going through a coke bottle: youtube.com/watch?v=-fSqFWcb4rE –  WernerCD Jul 13 '13 at 3:12
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Where does it go when you leave the light on? Surely it doesn't build up? That should answer your question :) –  Thomas Jul 13 '13 at 10:57
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3 Answers

Light travels at 300,000,000 meters per second. There is a very small period of time after switching it off where there still are photons from the bulb in the room. But these get absorbed/scattered by the wall and thus you don't see them. This what happens to all of the light that came out of the bulb significantly before you turned it off as well -- the photons (or EM radiation, take your pick) are no longer present in the room in their initial form. When the light hits the wall, some of it is absorbed, which heats up the wall ever so slightly (just how sunlight can heat things. For that matter, you can warm your hand even with a flashlight if you hold it in place fo a couple of minutes. You can do the same by placing your hand near a lightbulb). This heat dissipates through the wall. Also, some of it is scattered back with a shift in wavelength -- the electromagnetic radiation is still present in the room, but it is no longer visible. It could be radio waves/infrared waves or even ultraviolet waves. It's harmless, though. Eventually, it all leaves the room in the form of heat.

Remember, a light bulb doesn't "contain" light. It contains a filament, which glows when you pass electricity through it. This exactly what happens when you heat a metal -- the electrons get excited and start emitting visible light. Here, you are supplying energy to the bulb (in the form of electricity), and it gets heated, giving off light. For CFL bulbs the system is more complicated, but it still involves the excitation of electrons.

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+1, but I think the answer could be better with a short explanation of what happens when the photons are absorbed/scattered by the wall. The energy doesn't just disappear - dissipation as heat, radiation at non-visible wavelengths, etc. I think this could still be stated at a level a child could understand. –  Kyle Jul 12 '13 at 19:12
    
Assuming we're talking about incandescent (not fluorescent) lighting, it's not similar to what happens when you heat a metal. It is heating a piece of metal. The filament in a lightbulb is typically tungsten; the electricity which passes through the filament heats it due to resistance in the filament. –  Matt Ball Jul 12 '13 at 19:13
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If you want to demo flashlight warming with your kid; find an old incandescent light to do it with. This is one case where modern efficiency is working against you. –  Dan Neely Jul 12 '13 at 21:25
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@DanNeely Except that old incandescent lights themselves are hot. Very hot. It's quite easy to believe that the heat is from the bulb, not the light. –  Manishearth Jul 12 '13 at 21:33
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@DanNeely I know. However, to a child, it may be hard to explain that -- it's more likely in a child's mind for the explanation to be "hot thing makes air hot", not "light makes air hot" –  Manishearth Jul 13 '13 at 0:35
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If you want to explain it to your child, you could do the following home experiment.

  • Take a (cuttable) plastic box and cut some holes in it. Cut them all the way down to the floor of the box if you want the effect to be the most noticable and the analogy the most correct. Don't make too many holes and don't make them too big (try to mimic a scaled-down room with big holes for windows and tiny punctures for imperfections and absorptions.
  • Place the box in a sink with its top open.
  • Turn on the water supply - far enough to maintain a constant non-zero volume in the box. This is why you don't want to make too many holes, otherwise your water supply might not be able to handle it. Also, you don't want to be spoiling too much water. This situation corresponds to lights-on.
  • Then suddenly turn the water supply off. This obviously corresponds to lights-off. The water will leave the "room" through the "windows" and the "absorptions" of the big holes and the small holes. One sort of small hole in particular is worth mentioning to your child: your eyes. They are one of the many things in a room that absorb light. So in quite a literal sense, the light disappears into your eyes.

Explain to your child that this experiment is similar to the light in a room. As I mentioned, the light (water) disappears through the windows (big holes) and is absorbed by things in the room (small holes), like your eyes. In the case of light, the holes don't need to touch the room because the light - unlike the water - bounces around the entire room. It can reach higher places to escape (or be absorbed), while the water cannot.

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A nice idea, but it would be very easy for water to pool in the box, undermining the comparison. A cardboard box may actually be better, as it will absorb water, but of course it'll disintegrate soon too. –  Jon of All Trades Jul 12 '13 at 23:47
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How do you explain physics to a child while respecting his vision of the world. It is fascinating that most users ignored the question, while answering it or voting question and answers. The only ones who did listen were you and @TildalWave who is not a physicist (good joke). The question is: "when the switch is on, there is light everywhere, and when it is off you no longer see it; where did it go ?" How do you answer that question in child's terms without betraying science? I find this very interesting. –  babou Jul 13 '13 at 23:04
    
@babou Thank you :) It is difficult indeed. I tried to keep things as comprehensible for young children as possible with this answer, but of course you don't want to tell your child fairytales either. It largely depends on them. If they ask more and more in-depth questions after this experiment, you should go deeper into it and point out in which ways the analogy fails for example. And if they're really up for it, you can move on to e.g. the explanation by Manishearth, which probably also has a nice Aha-moment for a child with the a light bulb doesn't "contain" light. –  Wouter Jul 13 '13 at 23:29
    
There are really two points. One is properly reading a question and understanding it, which I too often fail to do myself (bad listener). The second one is understanding science and explaining it as simply as possible. I love it when I can do away with the math or unessential details, though it is not always well received by technicians, and sometimes pedagogically risky (sometimes clearer too). I said recently in another comment that I use math when I do not understand (which is overstating it a bit, but tries to make the point). It is also that my math is rusty. –  babou Jul 13 '13 at 23:57
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So when you turn the light switch on, you are effectively turning on a circuit, which has a light bulb (or several of them). What happens is that the current in the circuit then interacts with atoms in the filament and it causes them to heat up. Heat is just energy, so the atoms gain this energy, and they transition to higher energy levels. When these atoms go back down to their normal energy level, the energy is released as light. This is what we see.

So when you turn off the light switch, the light doesn't go anywhere, the current just stops flowing, which means that there is nothing to give the atoms energy so that they can transition between different energy levels.

The light that is in the room at the time you turn off the light is then scattered and absorbedby objects in the room, which is why the room goes dark

This link talks a bit about light bulbs in a simple context

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yeah, but when I looked at my hand with the light on, I could see it, then when I turned the light off, I could not. Therefore there must have been something on or near my hand, not just in the lightbulb. Where did it go? –  Brad Jul 12 '13 at 17:01
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You saw light being scattered off of your hand. The light originated from the tranisition between energy levels, and it bounces around and eventually reaches your eye. When you turn the light off there are no more photons being emitted, and instead they will be scattered and absorbed by items in the room. –  yankeefan11 Jul 12 '13 at 17:06
    
@JamesMaslek You should add this absorption of photons by objects in the room (in you comment) to you answer, as it is the main part of the answer to the question. –  Zorich Jul 12 '13 at 17:27
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@Brad Note that the photons when the light is on are continually absorbed by the surroundings, they bounce around and lose energy an become thermal. A 100 watt bulb heats the room with 100 watts like an electric heater except most of the energy of the photons is in the visual spectrum, and it degrades by collisions absorptions and re-emissions to thermal levels. –  anna v Jul 12 '13 at 17:48
    
Something like a tea sieve can work quite well for this demonstration. –  leftaroundabout Jul 13 '13 at 12:40
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protected by Qmechanic Jul 12 '13 at 20:45

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