Babinet's Principle in Electromagnetics: Why Does a Slot Radiate Like a Dipole? [Historical Corner]

This article attempts to address the question presented in its title, on the radiation of dipoles and slots and, in general, of complementary antennas. The endeavor begins with discussing Jacques Babinet and his far-from-trivial achievements in many fields. Then the original, scalar, version of Babinet's principle is given, together with a brief history of its porting to vector electromagnetics, and the article ends with a step-by step application to dipoles and slots.