An even better one - holding an image at the focal point of a lens produces its Fourier transform at the focal point on the other side of the lens[0]. It is use

An even better one - holding an image at the focal point of a lens produces its Fourier transform at the focal point on the other side of the lens[0]. It is used for "analog" pattern matching[1]. There is an interesting video explaining this on the Huygen Optics Youtube channel[2]. [0] - https://en.wikipedia.org/wiki/Fourier_optics https://en.wikipedia.org/wiki/Fourier_optics [1] - https://en.wikipedia.org/wiki/Optical_correlator https://en.wikipedia.org/wiki/Optical_correlator [2] - https://www.youtube.com/watch?v=Y9FZ4igNxNA https://www.youtube.com/watch?v=Y9FZ4igNxNA