Inverse Square Law & Guide Numbers

In physics, an inverse-square law is any physical law stating that some physical quantity or strength is inversely proportional to the square of the distance from the source of that physical quantity. <wikipedia>

Lightsource Intensity =1 / Distance squared

The Inverse Square Law defines the level of light from a fixed light source relative to the distance from the light source.

If the Light Source produces an output of light equal to X at a distance from light source to subject Y, at a lightsource to subject distance two times (twice) Y, the Light Source output will be equal to 1/4 of X.

Inverse Square Law <Geoff Lawrence Photography Tutorials>

Inverse Square Law and Guide Numbers <portraitlighting.net>

Interesting article on AlienBees by W.A. "John" Johnson

Calculating Range of Guide Numbers article

Giude Numbers = Flash-to-subject distance * f/stop
At 10 feet:
f * 10 = GN
f/8 * 10 = 80