Louis de Broglie, duc de Broglie, was born 115 years ago, on August 15th, 1892. His most famous discovery was a combination of Planck's "E=hf" formula with Einstein's "E=mc^2" formula.
More concretely, he realized that there is a (frequency f) wave associated with a (mass m) particle. Quantum mechanics would probably be born anyway, along the alternative matrix ideas of Heisenberg, but the waves remain the easier method among the two to imagine what's going on for most people.
Yesterday, we celebrated the anniversary of Hans Christian Ørsted. See a much more detailed story about this principal Danish co-founder of electromagnetism by Stefan.