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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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Old Nov 24th 2018, 05:25 AM   #4
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But you are talking about the intensity of the light that falls on your eye or other light collector- and that is the surface of the light sphere.
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Old Nov 25th 2018, 05:40 PM   #5
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Intensity is related to the number of photons reaching a surface, per square meter per second.

Using the cube of the radius would give a result related to the total number of photons emitted by the source in the time taken for photons to get from the source to the surface of a sphere of that radius.
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Last edited by Woody; Nov 26th 2018 at 02:52 AM. Reason: add "square" to units
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