Physics Help Forum Inverse square law?

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 Nov 10th 2018, 10:16 AM #1 Junior Member   Join Date: Nov 2018 Posts: 2 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? Tomthumbster likes this.
 Nov 10th 2018, 11:22 AM #2 Senior Member   Join Date: Aug 2010 Posts: 403 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. topsquark likes this.
 Nov 10th 2018, 11:51 AM #3 Junior Member   Join Date: Nov 2018 Posts: 2 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.
 Nov 24th 2018, 04:25 AM #4 Senior Member   Join Date: Aug 2010 Posts: 403 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.
 Nov 25th 2018, 04:40 PM #5 Senior Member     Join Date: Jun 2016 Location: England Posts: 845 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. __________________ ~\o/~ Last edited by Woody; Nov 26th 2018 at 01:52 AM. Reason: add "square" to units

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