In a brightly lit room, if the lights are suddenly turned off, the room will go dark almost instantly. We want to ask: where did all that light go? The short answer is that the light is all absorbed by the room.
When light hits a material, only three things can happen: it either passes through the material, is reflected by the material, or is absorbed by the material. As to which happens, a lot depends on the properties of the material. For example, a wall made of paper will transmit more light than a concrete wall, and a white wall will reflect more than a wall painted black.
Suppose we are in a room about 10 meters wide, that room is painted white, and the paint reflects about 90% of visible light. This means that it still absorbs 10% of the light every time it reflects light, so after one reflection, there will be only 90% of the light left. After two reflections, only 81% of the light is left, and after three reflections, about 73% of the light is left. After 7 reflections, only about 47% of the light remains, half of which is already absorbed. After 50 reflections, only 0.5% remains.
The speed of light is 300 million meters per second, and it can travel through a 10-meter room in 30 nanoseconds. If we assume that the reflection is instantaneous, that means it only takes about 1 microsecond to reflect 50 times and be almost 100% absorbed. To the human eye, 1 microsecond is basically an instant, which is why the room dims immediately after turning off the lights.
From this, we naturally thought of another question, what would happen if we were in a room composed of mirrors? In fact, a typical mirror is only about 99% reflective. That may sound like a lot, but even if only 1% of the reflected light is absorbed each time, it only takes about 15 microseconds for the light to be almost completely absorbed. So, for us, the mirror room is also darkened in an instant.
But, with physics, we always think about ideals, what would happen if we had perfect mirrors that could achieve true 100% reflectivity? Is it possible to build a room that is always bright? If that room really consisted of 100% reflectivity mirrors, that room would explode very quickly.
It's a simple fact that light has momentum. When light illuminates something, that light exerts a force on it, albeit a small force. Suppose a light bulb emits a photon, which then bounces off the mirrored wall and gives the mirrored wall momentum when its orientation changes. Then the photon is reflected back from the back wall, and a second reflection is performed on this mirror wall, and so on, there will be countless reflections. If the light has been on for a while, it's not the only photon hitting the wall. Even if that light is only on a few times, if that light isn't absorbed, there's a billion or so doubling of radiation pressure in a matter of seconds.
Assuming the room doesn't explode, and the walls of the room are free to move and expand as the light pushes, what happens when the lights are turned off in this case? Even if no light is absorbed, the room will actually darken again, and that's because light also has to obey the conservation of momentum. Give some of its momenta to the wall as it reflects off the wall and makes the wall move, and it loses momentum.
For most things, when they lose momentum, they slow down, but light doesn't, light always travels at the speed of light. So, when light loses momentum, its frequency becomes lower and its wavelength becomes longer, which is a redshift. As this happens with each bounce, our light eventually redshifts out of the visible range, eventually becoming radio waves of infinite wavelengths with negligible energy. So, the room darkened again.