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 deduced that light is a wave front spreading in all directions with each point the origin of a new little wavelet. Behind the wavefront, wavelets interfere to cancel out, as the trough of one denies the crest of another, so the resulting forward moving wavefront acts like a ray of light at a distance (Figure 3.1a). Huygen’s Principle, that each point of the light wavefront is a new wavelet source, explained reflection, refraction and diffraction. It followed that light is a wave.
In contrast, Newton observed that light travels in straight lines, rather than bending round corners as sound waves do when we hear someone talking in the next room, so he deduced that light was made of corpuscles that traveled in straight lines to match the optics of the day. His particle model explained only reflection and refraction (Figure 3.1b) but for some reason carried the day. It then followed that light consists of particles that move in straight lines.
Two hundred years later, Maxwell, using Faraday’s idea of a field, wrote equations that described light as an electromagnetic wave, based on a mechanical model of rotating vortexes. The equations worked, so they were quickly accepted, and this seemed to settle the matter that light was a wave.
However Maxwell’s equations assumed that light waves travel through a luminiferous ether that carries it, so the speed of light should vary according to its direction relative to that ether. But the Michelson-Morley experiment then showed that it didn’t, so there was no physical ether for light to travel relative to. Then Einstein equally convincingly argued from the photo-electric effect that light comes in particle-like packets called photons. The result was two theories, that light was waves and that it was particles, both of which had some degree of support.
In the history of science, the theory of light has swung from Huygens’ waves, to Newton’s corpuscles, to Maxwell’s waves, to Einstein’s photon packets with no clear winner, so modern physics finally gave up. It now concludes that light is wave and a particle, 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 miracle of wave-particle duality essentially enshrines our ignorance. Physical realism can explain light as a particle, or as a wave, but it can’t explain how it can be both.