Intensity: 4∏D2 vs. ∏r2 Explained

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davev
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Can someone please explain why the are used is 4∏D2? Shouldn't it just be ∏r2?
 
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The intensity spreads out over the surface of a sphere. Your formula is for the area of a circle.
 
sandy.bridge said:
The intensity spreads out over the surface of a sphere. Your formula is for the area of a circle.

That makes much more sense! Can't believe I totally missed that. Thank you!