Wave Nature

To resolve the difficulties of the corpuscular theory, the Dutch physicist Christiaan Huygens (1629-1695) proposed in 1678 that light is fundamentally a WAVE -- specifically, a wave caused by the vibration of particles of a medium, exactly as sound is a vibration of air. Since light also travels through the vacuum of space (evident from starlight and sunlight reaching us), Huygens further proposed that a hypothetical, all-pervading medium called ETHER fills even seemingly-empty space, and that it is the vibration of this ether that constitutes a light wave. (This hypothetical 'ether' has nothing to do with the ether GAS used in chemistry -- it is purely a proposed medium for light's vibrations.) No direct evidence for this ether was ever found, which made the wave theory difficult to accept at first.

Crucially, unlike the corpuscular theory, Huygens' wave theory correctly predicted that light should travel SLOWER (not faster) in a denser medium, matching what is actually observed. Once the nineteenth-century discoveries of interference, diffraction and polarization -- phenomena that the corpuscular theory simply had no mechanism to explain, but which followed naturally from wave behaviour -- were made, the wave theory of light became firmly established as the correct picture, alongside its ability to also explain the older, familiar laws of reflection, refraction and image formation. …