"Impedance" is a measure of how a system resists the inflow of energy. Often, a system can be changed to accept energy more efficiently, by adding an element called an "impedance matcher." Most people have never heard of an impedance matcher, but once you know about them, you begin to see them everywhere: in the shape of a trumpet, the anti-reflective coating on a lens, and the foam spikes on the inside of a recording booth.

A familiar example of an impedance matcher is the transmission of an automobile, which couples the energy from the engine into the wheels by converting the relatively fast rotation of the engine into the slower, stronger rotation required to propel the car. The transformer on an electric pole solves an analogous problem, converting the high-voltage electricity on the transmission lines to the high-current electricity required to power your home.

Some of the most interesting impedance matching occurs when energy comes in the form of a wave. In a swimming pool, waves from a splash reflect off the sides of the wall. Because there is an impedance mismatch between the water and the wall, the wave energy is unable to couple into the wall, and so it reflects back. If you watch a vertical seawall, you will notice that the reflected wave adds to the height of the original wave, creating a splash that is almost twice the height of the wave.

On a larger scale, we can think of atmospheric carbon dioxide as an undesired impedance matcher, coupling the infrared light waves of the sun into the planet. Someday, we may decide to cool our earth by adding tiny particles of dust to our stratosphere, tuning the optical surface to reflect away a tiny portion the infrared waves coming from the sun. The impedance mismatch between the atmosphere and sunlight would create a kind of half-silvered mirror to keep us cooler.