Suppose we take a sheet of ordinary metal, make a narrow slit in it, and shine a
ID: 1383251 • Letter: S
Question
Suppose we take a sheet of ordinary metal, make a narrow slit in it, and shine a light beam through the slit onto a screen. The light beam will diffract from the edges of the slit and spread out onto the screen.
Now let's take an identical sheet, with an identical slit, but this time made of neutronium, and shine an identical light through it.
Will the image on the screen be the same? Or are there additional interactions with the electrons in the first sheet which aren't present in the second experiment, and which cause the images to be different in the two experiments?
Explanation / Answer
If the geometry of the sheets is the same, they will usually have very similar diffraction patterns. Textbook derivations of diffraction patterns only make use of the geometry; their equations do not involve, e.g. the dielectric constant of the material. They are using a simple model in which the only way light interacts with the material is that the material blocks it. This model is generally very accurate for the materials used in the experiment.
If you wanted a more refined picture, then the material could indeed affect the diffraction pattern. A simple example is to make the material transparent. Then the light would mostly pass right through, drastically altering the interference pattern.
Another drastic case would be if the photons were interacting incoherently with the material - e.g. flipping electron spins states. Then the electrons would contain "which path" information about the photons. This would destroy interference altogether.
Even neutrons can scatter photons, so there will always be at least some minor interaction, but the difference between diffraction from 1 micron slits in gold foil and a 1 micron slits in tin foil will probably be so slight as to be undetectable, simply because the interaction can be modeled well enough by saying that the slit itself is radiating coherently while the surrounding metal is not radiating at all.
If you really wanted to calculate the diffraction pattern accurately, you would need to account for the way light interacts with the metal in detail, but you would also need to account for the exact atomic structure of your slit and the exact shape of your wavefront and the exact density of the gas it's traveling through and the exact position of the screen you're observing on and the exact efficiency of your detectors, etc. My guess is that in most typical cases the error in being able to construct the geometry you want and to get a really pure wavefront dominates over the error due to the interactions of light with the material.
Finally, in x-ray diffraction, the material is not only important, but is the object of study. When we use light with wavelength of a few Angstroms to a few nanometers, we can see diffraction off of the atomic structure itself. The atoms become the "slits". This is a major source of our knowledge of the structure of crystals.