Visualizing the Inverse Square Law

I recently lost a bet with a friend and fellow photographer. I now owe him a dollar because I couldn’t articulate the inverse square law correctly. The inverse square law, as it applies to photographers, is all about the falloff of illumination from a light source. A few days later he sent me an interesting hand-drawn illustration to help visualize the law, and I thought it was so unique, and allowed visual thinkers to see the situation in a unique and clear way, that I decided to share it here. Here’s what’s special: I’d always thought about light moving in a straight line and getting less powerful as it traveled, but I’d never thought about the inverse square law as a function of the same amount of light spread across a greater area. A subtle difference, but a big one in terms of my understanding. Because of that spreading, there is exponentially less light within the same square foot of real estate as you move farther from the source. The "light as water" concept kind of applies here: If you imagine a bucket of light spilling out of a source, there’s significantly more "water" a foot away than there would be two feet away, or four, or eight, and so on. As that puddle of light spreads out, it thins—and that ultimately creates the light falloff that the law defines. With every doubling of the distance from the source, light will fall off not just twice but four times—i.e. there’s four times less light because the same quantity of light is spread out over four times the area. Ultimately, every doubling of subject distance from light source equates to a two-stop drop in brightness. And now I can visualize that in another simple way thanks to the drawing of my friend. Maybe I should pay him that dollar after all. 

Leave a Comment