In the seventeenth century, Huygens noticed that light beams at right angles pass through each other so they must be waves, as if they were objects like arrows they would collide. He concluded that light was an expanding wave front spreading out in all directions, with each strike point the center of a new little wavelet. He proposed that as the trough of one wave cancels the crest of another, the wavelets interfere as they spread. The result is a forward moving envelope that at a distance from the source acts like a ray of light (Figure 3.1a). Huygen’s principle, that each wave front point is a new wavelet source expanding in all directions, explained reflection, refraction and diffraction. In contrast, Newton’s idea of bullet-like corpuscles traveling in straight lines explained only reflection and refraction (Figure 3.1b) but his simpler view carried the day.
Two hundred years later Maxwell wrote down the equations of light as a complex wave but then Einstein equally convincingly concluded from the photo-electric effect that it comes in particle-like packets called photons. So the theory of light has swung from Huygens’ waves to Newton’s corpuscles to Maxwell’s waves to Einstein’s photon packets. Then physics finally gave up and concluded that light is wave and a particle, although no-one can clarify how such a “wavicle” is even possible. Three centuries after Huygens and Newton, we still don’t know whether light is a wave or a particle.