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Old Nov 10th 2018, 11:16 AM   #1
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Inverse square law?

If the volume of a sphere increases as the cube of the radius, and light propagates as a sphere, why does the intensity of light decrease as the square of the radius, rather than as the cube?
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Old Nov 10th 2018, 12:22 PM   #2
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Because light does NOT stay in the interior of the expanding sphere as it propagates. It spreads through space on the surface of the sphere which increases as the square of the distance.
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Old Nov 10th 2018, 12:51 PM   #3
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Thanks

Thanks, HallofIvy - I guess I was thinking of a continuous light source, rather than an instantaneous burst, and that's what led me astray.
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