INTRODUCTION

Everyday experience gives us two very different pictures of how energy and matter move about. A particle -- a marble, a grain of sand, an atom, an electron -- is a tiny, localized concentration of matter that occupies a definite position at a definite instant. A wave -- a ripple on a pond, a sound wave, a light wave -- is a broad, spread-out distribution of energy that is not pinned down to any one place or moment. Both, however, share the ability to carry energy and momentum from one place to another, and classical physics built two entirely separate toolkits around them: particle mechanics for objects like planets and bullets, and wave optics for light and sound.

For a long time this separation looked complete. Electromagnetic radiation was firmly classed as a wave because it shows interference, diffraction and polarization -- phenomena that only make sense for waves, where two disturbances can reinforce or cancel each other. Electrons and protons, on discovery, were firmly classed as particles because they carry a definite mass and a definite charge. But two nineteenth/twentieth-century puzzles -- the shape of the radiation given off by a hot (black) body, and the photoelectric effect -- forced physicists to accept that electromagnetic radiation also behaves, under the right conditions, exactly like a stream of particles. This unit works out that particle nature of radiation in detail (through the photoelectric effect and Einstein's photon), and then turns the idea around: if radiation, ordinarily a wave, can behave like particles, can matter, ordinarily particles, behave like waves? De Broglie's matter-wave hypothesis, confirmed by the Davisson-Germer experiment, answers yes. The unit closes with X-rays, a penetrating radiation produced when fast electrons are suddenly stopped, whose spectrum is best understood using exactly this particle (photon) picture of radiation.