What is the actual explanation of diffraction?

[jeebs]

I've done plenty of problems over the course of my degree that involve waves passing through slits of widths comparable to their wavelengths, and spreading out. As far as I'm aware though, I haven't been told what is actually going on there, just that it does happen.
I know that we can accurately predict where the interference fringes on a screen infront of some slits will appear, and to do that we use Huygen's principle to treat every point on the wavefront as an individual source of new waves. We then work out where the individual contributions add and subtract from each other at the screen for our fringes.

However, is this really what's going on or is this just a useful trick that happens to help us solve this problem? (*more broadly, is ALL of physics just useful imaginary tricks to help us solve problems, if you get what I mean by that?)
What made Huygen come up with this explanation? It doesn't seem obvious to me, it's just something I've accepted blindly for the sake of exam marks.

I was thinking that maybe it could be a result of the position-momentum uncertainty principle. A wave (well a particle here really) passes through a slit, so we can determine its position parallel to the plane of the slit accurately - at some stage on its journey we know its within this narrow slit region. That means its momentum in this plane would become uncertain so it ends up deflecting off its original path, right? Is this a valid way to think of what's happening?
If it is, how does that work in the context of large scale, everyday situations (where QM effects aren't apparent) like water waves passing through a barrier? We can't use wave-particle duality there...?

 

[Born2bwire]

I've seen people use the uncertainty principle to explain the diffraction of a wavefunction but I do not think that it is a satisfactory explanation (how do you use it to explain edge diffraction where the "slit" is a semi-infinite interval?). Basically the diffraction comes up in quantum mechanics because quantum mechanics is a wave equation and so it exhibits the same wave behavior that we see in macroscopic waves under similar conditions.

I'm not sure what kind of explanation you would be satisfied with. One can come up with all kinds of explanations. Equivalent source theories are popular. For example, similar to Huygen's principle, is that we can treat the problem of waves impinging upon a boundary as inducing secondary wave sources on (and inside) the boundary. These sources enforce the boundary conditions and you can then think of the problem as nothing but vacuum with your incident wave and your induced sources. In this context, diffraction comes about because you have a truncation in the line of sources and this affects the resulting interference with the incident wave. The truncation is basically like leaving a single line source at the edge and so this line source sends out a cylindrical wave and we can see that in the diffraction pattern (and we can also see that in the rough calculations where to find the maxima and minima of a diffraction pattern you treat the edges as "sources" and find the positions of interference).

But underlying all of this is just the boundary conditions of the problem. A scatterer forces a set of conditions that must always be true upon any incident wave. In the case of an edge, we find that these conditions require the wave to diffract. As for a specific physical reason, that depends upon the actual wave. For electromagnetic waves, the idea of induced sources that I stated above works (since electromagnetic waves do induce physical currents in materials). Doesn't make too much physical sense when we think of water waves though. Perhaps in that case one could make a convoluted explanation as to the physical reasons (say viscosity, etc.) that water waves exhibit these boundary conditions.

 

[Drakkith]

A water wave and an electromagnetic wave are only similar in that they have "wavelike" properties. The only reason we call light a wave, is because it behaves like the classic water wave would in certain conditions. A water wave is made up of large numbers of particles, while an electromagnetic wave is not.

As far as wanting to know the actual explanation of diffraction, i suggest getting a book called Absolutely Small, by Michael D. Fayer. It explains quantum physics, and especially the wavelike properties of matter very well without getting into complex math. I'm in the middle of reading it and it very enlightening.