Causes of Diffraction Patterns

[Nacho]

Take a two-slit diffraction experiment. From what I've read or gathered, it is supposed to be caused by the geometry of the setup, and the frequency of the light (maybe a few more things). But, it is talked about in terms of the geometry, and no cause attributed to interactions between the light and the corners of the slits. I've even had someone tell me it couldn't be an interaction between the light and corners of the slits.

And even using electrons instead of light, it's still talked about the same; as the geometry and not about the electrons interacting with the substance of the slits.

But, when you probe a substance with an electron source, say like an electron microscope, it's clearly (to me at least) talked about in the sense of the electrons interacting with the substance being probed, and that the cause of the diffraction pattern.

I find these two things inconsistent; one an interaction and the other not. How is it again that we know (if I don't have my facts mistaken) the diffraction pattern of a two-slit experiment is not caused by interactions between the light/electrons and the sides of the slits?

 

[Crosson]

Do you know Hyugen's principle? It is difficult to understand in words, but easy in pictures, so search the web. It explains diffraction completely.

Basically, think of single slit diffraction as two slit interference, with infinitely many slits. What I am saying is that the single slit "acts like" infinitely many, infinitely small slits that interfere normally.

 

[Nacho]

Crosson,

I'm not real familar with Hyugen's principle. Thanks, I'll do a web search.

But how does that answer in the case of eletron microscopy? I guess it might be a matter of where the detector in it is located .. in front of, behind, or all around the sample being probed?

 

[develish16]

look guys. Diffraction simply has no real justified reason. Thats why it is a phenomenon!

 

[Andy Resnick]

The geometry *is* the interaction. In the plane of the slits, the electromagnetic field is confined to a specific region of space, given by boundary conditions that ultimately reflect the material properties.

Diffraction and scattering are the same thing.

 

[NWH]

Diffraction is awesome... :)

 

[wlminex]

Nacho: Certainly light interacts with the edges of slits. The interaction actually 'refracts' those photons, electrons, etc. that 'hit' the very thin slit edges. Refraction is induced where photons hit very thin parts of the slit (ideally monoatomic +/-thicknesses). Photonic energy is transferred to outer electrons of the slit material atoms, raising their (atom electron) energy levels. As the electron energies fall-back to their 'normal' state, the excess energy is emanated as secondary phtonic energy - probably of a frequency close (or harmonic?) to that of the impinging photon. So the poorly-understood "bending of light" around slit edges is dominantly a refraction process. The process does not work well when slit material thickness exceeds the ability of the impinging photon to penetrate the material completely. This explanation 'works for me' . . . no one has been able to explain this ever since Young first discribed the diffraction phenomenon. I have a brief write-up on this "theory", send me an email and I'll forward it to you. Please nominate me for the Nobel Prize in Physics! . . . haha! Regards,

Bill