The Inverse Square la w
This law states that if the distance between the light and subject is doubled, they'll receive a quarter of the light. You might think moving a light back from one metre to two metres would halve the strength, but it actually quarters it. To compensate, you’d either need to increase the power output by four times (as in the diagram) or allow for two extra stops of aperture or ISO. You can see the Inverse Square law in action in step 2 where, because of the change in distance ratios between light, subject and background, the background is made much darker when the flash is close to the face.