> Another is that far-field (i.e. long-distance; “Fraunhofer”) diffraction of coherent light on a semi-transparent planar screen gives you the Fourier transform

> Another is that far-field (i.e. long-distance; “Fraunhofer”) diffraction of coherent light on a semi-transparent planar screen gives you the Fourier transform of the transmissivity (i.e. transparency) of that screen Ooh, yes, I’d forgotten that! And I’ve actually done this experiment myself — it works impressively well when you set it up right. I even recall being able to create filters (low-pass, high-pass etc.) simply by blocking the appropriate part of the light beam and reconstituting the final image using another lens. Should have mentioned it in my comment…