An incandescent light bulb converts electrical energy into light and heat (a lot of heat--a typical 100 watt bulb gives off 5-7 watts of visible light, the rest is just heat and IR). In an incandescent bulb, a current passes through a tungsten wire in an atmosphere of inert gas (argon, krypton, or bromine and a little nitrogen). The current going through the tungsten wire heats it up to thousands of degrees, and causes it to glow brightly.

Light bulbs are rated in several ways. One way is to measure the electrical power the bulb uses in watts. But this is only the amount of power it takes up, not the amount of light given off. Candlepower, though, is a measurement of light power given off by the bulb. The amount of light given off by a candle flame is about 1 candlepower (obviously). Another measurement of the amount of light given off by a light bulb is a lumen. If you have a light source giving off 1 candlepower of light, and you have a square 1 foot by 1 foot, and it is curved so that all points are 1 foot away from the source, then 1 lumen of light is on it. If this square is part of a 1 foot radius sphere surrounding the source, then you could find the surface area of the sphere (4pr²) and find that about 12.56 lumens are given off by the source. If you were to have a 2 foot radius sphere, illuminated by the same light source, then in 1 square foot of that sphere, there would be ¼ of a lumen falling on that curved square. This is the Inverse Square Law, that is, the intensity of radiation is inversely proportional to the square of the distance from the source. Doubling the distance of an object from a radiating source will decrease the amount of radiation falling on the object to ¼ as much. But the amount of total energy falling on the 1 foot diameter sphere and 2 foot diameter sphere from the same source is equal, just a different density.

A picture of some incandescent light bulbs are shown above.

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