My name is Alexander FufaeV and here I write about:
Babinet's Principle: How to Predict Diffraction Patterns
The Babinet principle states that there is no difference in the light diffraction pattern if an equally wide wire (opaque obstacle) is used instead of a slit (transparent obstacle) - or vice versa.
In mathematical terms, Babinet's principle can be expressed as an equality between the amplitudes of the diffraction waves for the opaque and the transparent object. Thus, if one knows the diffraction at one object, one can predict the diffraction at the complementary object by applying Babinet's principle.