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Daniel Petka

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- TL;DR Summary
- The key of Fourier Optics: Why does a Plane Wave contribute to just a Single Point in the far field? (the plane wave goes through an aperature => actually a superposition of plane waves) I'm afraid my Fourier Optics profs doesn't quite get it either. (source: Fundamentals of Photonics, Saleh/Teich)

If the distance between the input and the output screen d is large enough, then for a plane wave (with some spatial frequencies vx and vy) at the input, the spot at the output will be a point. But if the plane wave is confined (aperature size b in the picture), it's no longer a plane wave... but a sum of plane waves. The proof here mentions the method of stationary phase and a phase factor that goes to zero. Sure, the math works out, but I still don't get intuitively. The only think that makes sense is to treat the plane wave as a gaussian laser beam (and so the dots are also gaussians). But this is not consistent, since a laser beam consists of many plane waves at different angles and here it's clearly mentioned that just

**one**plane wave / spatial frequency contribute to a given point... Thanks a lot for any insight!

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