Incandescence

­ ­Have you ever seen a heated horseshoe? Maybe at a blacksmith shop or on TV? If so, you know that if you get a horseshoe hot enough, it starts to glow. If you get its temperature up to 1,500 degrees F (800 degrees C) it will glow with a bright red color -- you see this temperature all the time in the coils of an electric stove, oven or toaster. In these appliances, electricity heats a coil or wire hot enough to make it glow. If you get the temperature up to about 4,500 degrees F (2,500 degrees C), you get a very bright yellow (nearly white) color. That's the temperature of a normal light bulb filament.

When something produces light because of heat, it is said to be incandescent. Anything that you heat up will glow, but different materials are better or worse at producing light as they are heated. Steel is a pretty good producer of light. Glass is a very poor producer. If you heat glass it will glow, but it gives off much less light than the same volume of steel. In the 1800s, theaters used lamps that heated a block of calcium oxide (lime) with a torch. This, by the way, is where the term limelight comes from. They used lime because it has a high melting temperature, so you can heat it to a white glow without the block melting (iron melts at 2,800 degrees F, while lime melts at around 4,600 degrees F). Lime also is a good producer of light.