Thanks Gary.
There's something I'm still not understanding about all this.
In the "basic" double-slit experiment described on the wikipedia linked to above, it's a coherent light source - single-phase, single frequency, in effect, a stream of photons emitted at a steady rate, yes? I imagine this would be a very narrow beam, i.e. with a diameter of 1 photon. This light source is directed at a surface with two slits in it.
Here's where I'm stuck:
Are the two slits narrower than the diameter of the coherent light stream (i.e. is 1 photon hitting both slits simultaneously)? In the video "Simulation of a particle wave function" (on the right hand size of the same webpage referred to above) this appears to be the case.
These must be very small slits, each less than half the diameter of a (massless) photon when allowing for the material between them!
The image I have in my mind of these photons is of a stream of ping-pong balls being fired out of a nerf gun. I think my classical training is showing!