Suppose that a flashlight show in your face and then was moved away at a constant rate of 2 feet per second. It is fact that the rate at which the intensity, I, of the light decreases is inversely proportional to cube of the distance, R, from the source of the light times the rate at which the distance increases. When the light is 10 feet from your face, the intensity is changing by 5 lumens per second. When the light is 10 feet from your face, the intensity is changing by 5 lumens per second.
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