For some time I've had a vague feeling that the visible double slit result isn't so much a consequence of the light (or whatever) that passes through the slits (which are obviously 'open') but is in reality a consequence of the paths that absent. Enter recently discovered (by me) Babinet's Principle with complementary screens where the slits are filled in one and are gaps in the other. [Jacques Babinet 1794-1872] https://www.physics.princeton.edu/~mcdonald/examples/babinet.pdf The principle may (or may not) be exact. Assuming exact: On the one hand it is possible that diffraction at one edge is cancelled by 'dark' diffraction at the other edge - alternatively diffaction is a sort of non-phenomenon caused by the absence of light through the opaque region. Does anyone know anything about this or have any thoughts?