Quantum Electrodynamics (QED) is a theory which combines quantum mechanics and special relativity to describe the interactions of electrons (and other charged particles) among themselves and with particles of light (photons). QED predicts very accurately the motion of an electron in presence of a magnetic field. The agreement between theory and experiment can reach eight significant figures which is considered as quite remarkable. In 1965, R. Feynman, J. Schwinger and S. Tomonaga were awarded the Nobel Price for their work about QED, done around 1950.
The first two chapters of Feynman's book QED explain the ``recipe'' to calculate the location of the dark spots in experiments with a few narrow slits (interference and diffraction).
One triumph of the quantum theory of electrons and photons is that it explains the periodic table of elements. For details, see p. 113 of QED and chapter 19 of the third volume of Feynman's Lectures.