1. Rays, waves and photons

It has long been known that light propagates in straight lines: the light rays that pierce through clouds or foliage in a dusty atmosphere are striking proof of this, giving the impression of a fan of rays from the sun. For many years, the particle and wave theories of light were the only ones in competition (Newton being the champion of the former and Huygens of the latter), and it wasn't until the twentieth century that quantum theory reconciled them, although there are still some difficulties of understanding.

The essential tool for optical design is still the old, empirical theory of light rays (reflection, refraction), but to fully understand the fundamental phenomena involved in image formation, it is essential to bring in the wave-like aspects of light (diffraction, interference), to take into account the limitations they impose on optical systems....