QR3.6.1 A Wave Moves

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

If light moves as a wave, it should bend round corners as sound waves do when we hear people talking in the next room and indeed it does. In 1660 Grimaldi found that light does bend but by less due to its shorter wavelength. How then can a wave move in a straight line? Figure 3.13 shows how the photon wave power varies along its directional axis so it is more likely to exist at the thicker sections. The result of detecting photons by screens at different distances confirm this as they aren’t in 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! Only the average measurements are a straight line.

Figure 3.14. Detection of a photon of light

If light only travels in a straight line on average, why doesn’t it sometimes “bend into the shadows”, to show us a torch beam from the side? On the other hand, if light is photon particles traveling in a straight-line path, as optics suggests, how does it find its way?