Monday, September 24, 2012

When the Lights Go Out

One of my readers posed the question: When I turn off the light in my bedroom at night, where does all the light go? Before we answer this, we need to make a few assumptions. Let’s assume that the bedroom is a perfect container: It has no windows for light to escape from and no cracks around the door so light can’t escape there either. Also, let’s say the walls are perfectly solid, and consist of a regular structure of atoms. Imagine a grid of hard spheres laying next to  each other. This is the surface of the walls. 
   First, we need to realize that light is a form of energy. While the light switch is on, it closes an electric circuit and electrons flow through the light bulb. The light bulb converts the energy from the electric current to light energy in the form of photons. Photons are tiny packet of electromagnetic energy and momentum. When you turn the light off, the circuit is broken, the energy stops flowing, and the light goes away. But where does it go?
   The photons travel across the room at the speed of light. When a photon hits the wall, its energy and momentum is either absorbed by the atoms in the wall, or are reflected to another wall where it again may get absorbed or reflected. One of the fundamental concepts of physics tells us that both energy and momentum are conserved, which means that an atom will get a small kick from absorbing a photon. It will move, and kick against its neighbor, etc. 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. This is what is meant by having a temperature. 
   If the wall were at absolute zero, these atoms do not move and are simply at rest, each one just touching the next. By saying that the wall has a temperature, we are really saying that it contains thermal energy. This thermal energy is the random vibration of the atoms around their equilibrium point. Such a vibration can travel through the grid of atoms in the form of a wave. One atom pushes the next, which pushes the next, etc. 
   When you turn off the light switch, the process just stops—the bulb stops generating photons, and the last set of photons hit walls until they’re all absorbed, all within a fraction of a second.

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