Introduction

Light has puzzled physicists for centuries because no single model captures every one of its behaviours. This unit begins with the simplest and most practical picture: ray optics, which treats light as travelling along straight-line paths (rays) and leans on ordinary geometric constructions to work out how mirrors, lenses and prisms bend and focus it. This geometric approach is powerful enough on its own to explain reflection, refraction, dispersion and scattering, and it is the approach the rest of this unit is built around. It cannot, however, explain everything light does -- interference and diffraction only make sense once light is treated as a wave, the subject of the very next unit -- and at even smaller scales, when light interacts with matter, its particle-like (quantum) character becomes important, a topic left for graduate-level quantum optics.