system
Given the following post, respond as if you're commenting on Reddit:
human
Where does light go?

If I have an electric light switched on in a room it gives off light. Photons I presume. When I switch it off what happens to them? Where does that energy actually go?

The previous question may answer this but why doesn't light bounce around in the room forever?
gpt
When your light is switched on, electrons are being pushed along a Copper wire into a thin Tungsten wire, which is very resistive. The electrons in the resistor bump into the atoms as they flow around, which causes the material to heat up. When a material is hot enough (around 3000K) it emits a lot of light, through a process called blackbody radiation. Tungsten happens to have a melting temperature around 3700K, so it is a great material for light bulbs. There are many other types of light sources, but this is the simplest to understand. As long as the Tungsten wire is hot, it will give off light. When you turn off the light switch, the electricity stops flowing, and the Tungsten wire cools down.

The light emitted by the bulb travels in every direction, and hits the various objects in your room. The whiter the objects, the higher percentage of the original light is re-emitted each time they are hit by photons. The blacker the objects are, the higher percentage of light is absorbed and makes the object hotter.

If the room was filled with very smooth conductive surfaces like mirrors, the light would bounce around many times. Very clean mirrors are around 99.9% efficient, which means that 1 in 1000 photons are absorbed as heat and the remainder are reflected. Because photons travel at the speed of light, they will bounce around in the room a few thousand times at most and eventually all be absorbed. Light travels around 30cm per nano-second, so in a 6m wide room it would take 20ns to cross once and around 50-100us for almost all to be absorbed.

TL;dr : The light is all absorbed as heat by objects in the room less than a milisecond after it is emitted.