When I turn off the lights, where does all the light go?

by Thecat5001

Light is a form of energy, but when you turn the light off, the light goes away, so where does the energy go?

The short answer is: it gets absorbed by the wall as heat.

The longer answer needs a bit of a more detailed mental picture. The wall is a solid, which consists of a (fairly) regular structure of atoms. Just imagine a grid of hard spheres laying against each other. This is the surface of the wall. At absolute zero, these atoms do not move and are simply at rest, one just touching the next. Having a temperature means that the wall contains thermal energy. This thermal energy is a random motion of the atoms around their equilibrium point, they're basically vibrating. Such a vibration can travel rather far through the lattice in the form of a wave. One ball pushes the next, which pushes the next, which pushes ... etc. Such a wave is commonly called a 'phonon', because it is also the way in which sound can move through solids.

Now think of the light. Light consists of tiny particles called photons, not to be confused with the phonons in the wall. Each photon is a tiny packet of electromagnetic energy and momentum. If such a photon hits (an atom of) the wall, its energy and momentum is absorbed. Since both these quantities need to be conserved, it means the atom will get a little "kick" from absorbing the photon. It will move, and kick against its neighbor, etc etc. So basically the photon has been converted into a phonon.

If enough photons get absorbed, this will result in the wall warming up slightly. So the light gets converted into thermal energy in the wall.

It's rather analogous to a stone falling into a lake. The energy of the stone will spread out over the surface of the water in the form of waves. The water itself doesn't move much, but the waves can carry the energy quite far. Likewise, the atoms don't move much, but the energy/momentum from the photons can carry rather deep into the wall.