Something I always find pretty mind-blowing is that a physical thin lens can also perform a Fourier transform. This (generally) only applies to coherent light,

Something I always find pretty mind-blowing is that a physical thin lens can also perform a Fourier transform. This (generally) only applies to coherent light, so it's not something we're used to everyday, but it forms the basis of a lot of laser-based optical technologies. Combined with some other effects you can do "all-optical" signal processing [1,2], or generate create dynamic, "programmable" holograms [3] [1] https://ieeexplore.ieee.org/document/686739 https://ieeexplore.ieee.org/document/686739 [2] https://ieeexplore.ieee.org/document/6648413 https://ieeexplore.ieee.org/document/6648413 [3] https://en.wikipedia.org/wiki/Computer-generated_holography https://en.wikipedia.org/wiki/Computer-generated_holography