A week ago, while browsing the electrical aisle at the local home center, I stumbled across a light dimmer that claimed it saves energy over other light dimmers. Many years ago, light dimmers were simple variable resistors connected in series with the light bulb that limited the amount of current flowing through the bulb. In that type of circuit the amount of energy used is the same, because part of the energy will be expended by the variable resistor as heat, instead of as light from the bulb. Obviously, an “energy saving” light dimmer must be a little more complicated than a simple variable resistor.
The internet settles casual curiosities perfectly. For topics like this, a quick Google search is much faster than digging out the old physics book (that you probably sold for beer money, anyway). My search turned up this explanation, and an interesting website to browse when stuck in a hotel room on a business trip:
As it turns out, most light dimmers use a duty cycle principle to dim a light bulb. That means, the light bulb is actually flashing on and off sixty times per second (in North America), and the dimmer simply varies the amount of time the light bulb stays on. The duty cycle is controlled using a very simple circuit using a type of a semiconductor called a triac, a variable resistor, and a capacitor. In this type of circuit, a dimmed light bulb will use much less energy over a fully lit bulb, because it is turned off for part of the cycle. The dimming circuit uses much less energy, because only a very small amount of current flows through the variable resistor.