- 13,466

- 5,362

Again: A "laser beam" is to a pretty good approximation a coherent state and not a single-photon state. It's well described as a classical electromagnetic wave, and I don't understand what you mean by that this example shows something non-classical. It's just diffraction at an edge, which has been fully understood (i.e., beyond Kirchhoff's approximate solution) by A. Sommerfeld, who was an expert in the theory of optical diffraction. His optics textbook is simply marvelous:A direct observation that make me think "this can't be classical" is rather very trivial. Its large divergence of laser beam when it is partially blocked by a barrier versus small divergence of freely propagating laser beam. I don't see that this can be explained by Huygens principle (or bullet type particles).

A. Sommerfeld, Lectures on Theoretical Physics, Vol. IV (Optics), Academic Press (1954)

Huygens's principle has to be used with a grain of salt. The mathematically fully correct version is found through using the retarede Green's function of the wave equation (in 3D space!),

$$G_{\text{ret}}(t,\vec{x})=\frac{1}{4 \pi |\vec{x}|} \delta \left (t-\frac{r}{c} \right), \quad r=|\vec{x}|.$$

For details, see Morse&Feshbach, Methods of theoretical physics, Vol. 1.