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 but arrow-like particles would collide, so it must be a wave. He deduced that light is a wavefront spreading in all directions, with each point the origin of a new little wavelet. The wavelets behind the wavefront interfere, as the trough of one cancels the crest of another, so 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 and implied that light is a wave.
In contrast, Newton observed that light travels in straight lines rather than bending round corners as sound does when someone talks in the next room, so his optics described light as corpuscles that traveled in straight lines (Figure 3.1b). This model explained only reflection and refraction but it carried the day, so light was then seen as particles.
So it remained until two hundred years later, Maxwell used Faraday’s field idea to write down the equations of light as an electro-magnetic wave, based on a mechanical model of rotating vortexes. The equations worked and were accepted, so light was then assumed to be a wave.
Yet waves need a medium, so light would need an invisible ether to travel in space. The speed of light on earth should then vary relative to its direction, as the earth was spinning and moving around the sun, but the Michelson-Morley experiment showed that it didn’t, so light couldn’t be a wave because there was no physical ether. Then Einstein concluded from the photo-electric effect that light comes in particle-like packets called photons, so light was again made of particles!
The scientific answer to what light is has swung from Huygens’ waves, to Newton’s corpuscles, to Maxwell’s waves, to Einstein’s packets, with no clear winner, so physics finally gave up. It decided that light is wave and a particle, even though that seems impossible. Three centuries after Newton, we still don’t know whether light is a wave or a particle, and calling it a wave-particle essentially enshrines our ignorance. Science can explain light as a particle, or as a wave, but how it is both is inexplicable.