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So today I was asked a question which I found very simple yet I honestly couldn't explain (embarrassingly).

One of my mum's friends who knows I'm a scientist asked me "If I turn the light off in a room why does the light instantly disappear"

To phrase it up in a more scientific way :

If we took a box of light and sealed up the entrance to the box such that the photons can't escape, how come (since the photons are still inside the box) it appears dark?

My thoughts were to do with the source of the radiation being removed, but I have no idea why photons bouncing about inside the box do not light up the inside? Intensity maybe?

I guess its sort of like black body radiation approximated by a cavity with radiation inside and a pin small hole following Planck's model, but in this case we just close the hole rather than measure any emitted radiation.

Any ideas?

Edit Thank you all very much for your answers! I understand now. Hypothetically speaking if the walls were of a material that did not permit the adsorption of a certain frequency of the light would it just bounce around forever? (I actually have another "simple" question to ask, but I will ask it in a separate part later this evening. Thanks again :)

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  • $\begingroup$ A good popular level question. Ask yourself what happens to a photon (or a classical light ray) when it gets to the wall. $\endgroup$ – dmckee --- ex-moderator kitten Nov 4 '14 at 19:39
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    $\begingroup$ Essentially a duplicate of Why do light disappears the moment we switch off the source (inside the wooden box)? $\endgroup$ – Kyle Kanos Nov 4 '14 at 19:42
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    $\begingroup$ Keep in mind that the whitest white paint only reflects about 85% of the light striking it. In an all-white room, with light traveling at 186 thousand miles per second, a beam of light in the room will be reflected roughly a billion times in one second. $\endgroup$ – Hot Licks Nov 4 '14 at 23:23
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The solution boils down to examining what is meant by the term instantly in:

"If I turn the light off in a room why does the light instantly disappear"

All of the visible photons in the box cannot instantly disappear since the information about the source going out only travels at finite speed $v\approx c_\text{vacuum}$.

Once the source stops producing photons, the photons that are already in the box will continue traveling. Eventually each photon hits the side of the box. Some photons are absorbed by the box (warming it up), while others are reflected. These reflected photons continue traveling until they hit one of the other sides. Rinse, repeat.

Eventually there have been enough reflections that the number of visible photons in the box is zero.

So the photons don't disappear instantly. But it does happen very quickly, and it is certainly imperceptible to our silly human senses. In this way the light may seem to disappear instantly.

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As time passes there is a fast aborption of photons by the cavity walls (or the walls of the room) followed by the emission of photons of longer wave lengths (infra-red radiation) which we do not see. That's why it becomes dark.

Of course, if the cavity isn't completely sealed up there will be radiation loss to the universe and eventually it will reach thermal equilibrium.

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In a very short time, a light beam can bounce around the room many times. Each time it hits a solid surface a fraction is absorbed and the rest is reflected. It is either absorbed mostly by walls, floors and ceiling or bounces out the window faster than the eye can see. It does have a small role in heating the room and thus contributing to the IR radiation that the room emits.

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