Figure 17-4: The Inverse-Square Law Radiation 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) = 1/9 of that at d = 1.

(b: Corbis)