A Law of Light

Now you know more about Cepheid variable stars. Using the relationship between their period of varying brightness and their absolute magnitude allows scientists to make a good estimate of their brightness. How does this tell us how far away they are? It's simply a matter of light intensity and distance. The inverse-square law of light states that the brightness of a light source decreases by the square of the distance from the light source.

HUH?

Let's try that again. You know that the farther away you are from a light source, the dimmer it is. If you stand one meter from a lamp, it is a certain brightness. But here is the interesting part. If you move two meters away, the light is not half as bright as you might expect. It is one fourth as bright. And if you move three meters away, it is one ninth as bright. The light is dimming by the square of the distance away. Clear as mud? Perhaps this animation will help shed a little light on the subject. (Ouch!)

At twice the distance away, the 1000 watt bulb shines at only 250 watts, one fourth the original intensity. At 10 times (10x) the distance, the bulb is only 10 watts bright, one hundredth of its starting brightness.