The question of whether light is a particle or a wave has a long history. In the seventeenth century, Huygens observed that light beams at right angles pass through each other like waves, while arrow-like particles would collide. He concluded that light is a wavefront spreading in all directions, with each point the origin of a new little wavelet. Behind the wavefront, wavelets interfere, as the trough of one cancels the crest of another. The result is a forward moving wavefront that acts like a ray of light (Figure 3.1a). Huygen’s Principle, that each point of the light wavefront is a new wavelet source, explained reflection, refraction, and diffraction, so light is a wave.
In contrast, Newton observed that light travels in straight lines, rather than bending round corners as sound does when we hear someone talking in the next room, so he concluded that light consisted of corpuscles that traveled in straight lines, as in the optics of the day. His particle model explained only reflection and refraction (Figure 3.1b), but he carried the day, so light was then seen as particles that move in straight lines.
So it remained, until two hundred years later Maxwell, using Faraday’s idea of a field, wrote down the equations of light as an electro-magnetic wave based on a mechanical model of rotating vortexes. The equations worked, so they were accepted, which seemed to settle the matter that light was a wave.
However, Maxwell’s equations assumed that light waves travel by means of an ether, so the speed of light should vary relative to that ether, but the Michelson-Morley experiment showed that it didn’t. Light couldn’t travel in a physical ether, so it wasn’t a wave. Then Einstein showed, from the photo-electric effect, that light comes in particle-like packets, called photons. The evidence suggested both that light was a wave and that it was particles!
In history, the science of light has swung from Huygens’ waves, to Newton’s corpuscles, to Maxwell’s waves, to Einstein’s photons, with no clear winner, so physics finally gave up. It decided that light is wave and a particle, even though no-one can explain how that is possible. Three centuries after Huygens and Newton, we still don’t know whether light is a wave or a particle, and the current wave-particle duality essentially enshrines our ignorance. Physical reality can explain light as a particle, or as a wave, but it can’t explain how it can be both.