Optics

Optics as physics concerned with the manipulation and study of light and, more recently, the general study of electromagnetic radiation, has a history back to ancient Egypt, and systematic study to classical Greece. But physics has proved better able to manipulate light than to explain its fundamental nature.

‘Geometrical optics’ treats light as a bundle of discrete rays, tracing their rectilinear paths reflected from surfaces and refracted through transparent media. ‘Physical optics’ treats light as a wave. It explains the dispersion of white light into spectral colours, the bands and colour patterns of diffraction phenomena, and aspects of the absorption and scattering of light. Characterizing the way in which the physical aspects of light become the perceptual aspects of shape and colour joins physics, physiology and philosophy in the perennial question of the correspondence of our perceptions to the physical world itself.

A modern view of light describes it in terms of massless particulate photons. This ‘quantum optics’ treats the absorption and emission of light by matter; providing precise knowledge of matter’s inner structure, and the technology of lasers. Philosophically, quantum optics has led to the fundamental question: what is light? What is this natural entity which is created and destroyed in a particle-like way and yet propagates through space – and lenses, holes and slits – in a wave-like way? Experiments in which individual photons interfere with themselves make it hard to think of them as having unique paths. Experiments involving the correlation of photon properties threaten attempts to describe photons as having individual properties and interacting only locally.