Figure 10-6 The Inverse-Square Law (a) Energy from a light source illuminates an area that increases as the square of the distance from the source. Hence, the apparent brightness decreases as the square of the distance. The brightness at d = 2 is 1/(22) = ¼ of the brightness at d = 1, and the brightness at d = 3 is 1/(32) = % of that at d = 1. (b) The change in apparent brightness for a light source with constant luminosity as a result of changing distance is shown using an approaching automobile with headlights.