Shulou(Shulou.com)11/24 Report--

In a brightly lit room, if the lights are suddenly turned off, the room darkens almost instantly. We want to ask: where are all those lights? The short answer is that the light is absorbed by the room.

When light is incident on a material, only three things happen: it either passes through the material, is reflected by the material, or is absorbed by the material. Which situation will happen depends to a large extent on the nature of the material. For example, a wall made of paper will be more transparent 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, which is painted white and the paint reflects about 90% of the visible light. This means that it still absorbs 10% of the light each time it reflects light, so after a reflection, only 90% of the light will be left. After two reflections, only 81% of the light will be left, and after three reflections, about 73% of the light will remain. After seven reflections, only about 47% of the light is left, half of which has been absorbed. After 50 reflections, only 0.5% is left.

Light travels at a speed of 300 million meters per second, and it can travel through a room of 10 meters in 30 nanoseconds. If we assume that the reflection is instantaneous, this means that it only takes about 1 microsecond to reflect 50 times and is almost 100% absorbed. For the human eye, 1 microsecond is basically instantaneous, which is why the room darkens immediately after turning off the lights.

As a result, we naturally think of another question: what if we were in a room made of mirrors? In fact, the typical mirror has only about 99% reflectivity. This may sound like a lot, but even if only 1% of the reflected light is absorbed, it only takes about 15 microseconds, and the light is almost completely absorbed. So, for us, the mirror room darkens in an instant.

But when it comes to physics, we always think about the ideal. What happens if we have a perfect mirror that can achieve a true 100% reflectivity? Can you build a room that is always bright? If that room is really made up of mirrors with 100% reflectivity, then that room will explode quickly.

This is a simple fact that light has momentum. When light illuminates something, the light exerts a force on it, though the force is small. Suppose the light bulb emits a photon, and then it reflects off the mirror wall and gives the mirror wall a momentum when its direction changes. Then the photon is reflected back from the back wall, and a second reflection is made on the 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 that hits the wall. Even if the lamp is on only a few times, if the light is not absorbed, the radiation pressure will be doubled by about a billion times in a matter of seconds.

Assuming that the room does not explode and the walls of the room can move freely and expand as the light is pushed, what happens if the lights are turned off in this case? Even if no light is absorbed, the room actually darkens again, because light must also obey the conservation of momentum. When it reflects from the wall and causes the wall to move, give some of its momentum to the wall, and it loses momentum.

For most things, when they lose momentum, they slow down, but light does not slow down, and light always travels at the speed of light. So, when light loses momentum, its frequency becomes lower and its wavelength becomes longer, that is, a redshift occurs. As this happens with each bounce, our light eventually redshifts out of sight and turns into radio waves of infinitely long wavelengths with negligible energy. So the room darkened again.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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