Nature of Light

The question of what light fundamentally IS took centuries to settle, moving through three distinct pictures before arriving at the modern, dual understanding.

Corpuscular (particle) nature. René Descartes (1596-1650) first proposed, in 1636, that light consists of a stream of particles. Newton (1642-1726) developed this into a fuller theory: light is made of hard, elastic, massless corpuscles emitted by a source, travelling in straight lines unless acted on by an external force. Reflection was explained as an elastic collision of corpuscles with the reflecting surface, correctly reproducing the laws of reflection. Refraction was explained by proposing that a denser medium exerts a stronger attractive pull on the corpuscles near the boundary, accelerating them toward the normal — and different colours were attributed to corpuscles of different sizes. This theory made one sharp, testable prediction: since a denser medium was assumed to pull the corpuscles in and speed them up, light should travel FASTER in a denser medium than in a rarer one. When the speed of light in water and air was later measured directly, this prediction turned out to be exactly backwards — light travels slower in a denser medium — and the corpuscular theory could not survive the contradiction.

Wave nature. The Dutch physicist Christiaan Huygens (1629-1695) proposed, in 1678, that light is a wave rather than a stream of particles. Since a wave was, at the time, understood to always need a medium to carry its vibrations, and light clearly travels through vacuum too (starlight reaches us across empty space), Huygens had to assume a hypothetical, all-pervading medium called ether, filling even vacuum, and distinct from the ether-gas of chemistry. There was no direct experimental evidence for ether, which kept the theory on shaky ground for a long time. Matters changed decisively in the nineteenth century, once phenomena — interference, diffraction and polarization — were discovered that only a wave picture could explain; the corpuscular theory had no account of them at all, while Huygens' wave theory explained these AND still reproduced the laws of reflection and refraction and mirror/lens image formation, correctly predicting that light bends toward the normal on entering a denser medium because its speed there is LOWER — the opposite, and correct, prediction to Newton's. Wave theory was accepted as the correct theory on this basis. …