QR3.6.1 A Wave Moves

How light travels to a destination depends on whether it is a wave or a particle. Newton explained why he rejected Huygens’s wave view of light as follows:
For it seems impossible that any of those motions … can be propagated in straight lines without the like spreading every way into the shadowed medium on which they border.” (Bolles, 1999) p192
Figure 3.13. A photon probability of existence

He was correct that if light moves as a wave, it should bend round corners as sound waves do, but it turned out that it does. In 1660 Grimaldi found that light does bend but less than sound as its wavelength is shorter. This changed the question to how can a spreading wave travel in a straight line?

According to quantum theory, where a photon is detected depends on the power of the quantum wave. Figure 3.13 shows how the photon wave power varies along its direction axis, where it’s more likely to exist at the thicker sections.
Figure 3.14. Detection of a photon of light

Detecting photons by screens at different distances confirms this, as the results aren’t a perfect straight line but randomly spread about (Figure 3.14). A physical particle would have to travel in a zigzag path to explain this! When a photon moves, its maximum probability of existence is a straight line but the wave itself spreads in all directions!

If light only travels in a straight line on average, why are the straight lines of Greek optics so effective? The answer turns out to be not because light is made of particles but because it arrives at a single point, but first, let us continue the story.