Diffraction by a cat’s whisker
The diffraction patterns produced by a single slit of width w and a thin wire of the same width are exactly the same. This is called Babinet's principle and is based on the fact that the interference pattern produced by blocking out part of a wavefront depends only on the locations of the edges of the diffraction obstacle.
“Diffraction By a Cat’s Whisker”, The Physics Teacher, 38, 422 (2000)
The Physics Teacher