Schematic showing how light intensity (L) falls off with the square of the distance (R) - known as the inverse square law. At a distance R from a light source, the luminosity of the source is recorded as L. Nine photons (yellow dots) are seen intercepting the small square closest to the source. At a distance of 2R, the light intensity drops to 1/4L. And at 3R, the intensity is L/9. Each unit of area now has roughly one-ninth of the photons intercepted at distance R. This principle enables astronomers to determine the distance to certain sources by comparing their luminosities with others sources whose distance is already established.