If a crystal has diameter roughly 0.25mm containing unit cells (say cubes) of about 1 angstrom on a side, there are about 2.5 million unit cells in the diameter of the crystal. Bragg's law (n$\lambda = 2d\sin\theta)$ applies, and tells us that we would like to use wavelength on the order of 1 angstrom to probe the crystal.

But if $\lambda = 5000$ angstroms (a laser) it seems we could still get refraction of parallel waves separated by about 2500 angstroms, or about $0.25$mm/$2500$ angstrom $= 1000$ reinforcing refractions in the above crystal, considering only the one dimension. Hence my question--

Is it possible that while we cannot resolve structures smaller than a few angstroms without using x-rays, we might get information (albeit attenuated) about identical repeating structures (crystals) using longer wavelengths?


The images below may help clarify. Above is radial average intensity calculated in python for the laser diffraction of zircon image (below, with central beam edited out). One one hand I wouldn't try to correlate the calculation with XRD calculations, on the other hand there seems to be some non-random information in the image. If nothing else, this may clarify my misconceptions.

attached images

Edit 2

Am adding the

unedited photo of ZrO2.

unedited photo of ZrO2.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – Buzz
    Aug 26, 2021 at 0:22
  • 1
    $\begingroup$ physics.stackexchange.com/questions/661750/… is the question I asked about this conundrum. $\endgroup$
    – Gert
    Aug 27, 2021 at 14:41
  • $\begingroup$ To clarify: in theory a perfect crystal contains much larger identical subunits--say 5000 unit cells in diameter-- which could interact with longer light wavelengths. So Bragg's relation applies and we get what might be called 5000th order effects. In practice I think there are no effects and there is something wrong with the assumptions. $\endgroup$
    – daniel
    Sep 4, 2021 at 11:28
  • $\begingroup$ Based on the answer below it is likely the photo is of reflection, not diffraction. $\endgroup$
    – daniel
    Oct 6, 2021 at 12:56

1 Answer 1


The following isn't really an answer, rather a rebuttal of sorts.

Using visible light to get (weak) diffraction information about crystals

would of course a very interesting thing to do, which is why as a chemist it piqued my attention. But I fear that in accordance with common perception it isn't really possible and that which is on display in the question is something else.

I first ran a quick test with a monocrystalline Calcite crystal, about the size of my thumb. I shone a $532\mathrm{nm}$ ($8000\mathrm{mW}$) green pointer laser through it and obtained two weakened central beams and a completely random 'halo' of scattered light. No diffraction pattern whatsoever.

I then finely ground up some kitchen salt (chemically $\text{NaCl}$) in a mortar and pestle and loaded it into a glass capillary tube.

But trying to shine a laser beam through that is a waste of time: like most bulk transparent materials (glass, sucrose crystals, salt crystals etc) their ground up versions simply aren't transparent to VIS.

They would be to X-rays but that would yield circular diffraction patterns. Only monocrystallin materials would show the kind of diffraction pattern the OP shows in his question.

So I don't know what's causing the interesting pattern but I don't think it is diffraction related. But I'd like to know more about the OP's experimental set up to understand what might be producing the observed effect.

  • 1
    $\begingroup$ Thanks for your upvote. Look again at Bragg's Law ($\lambda =2d\sin\theta$ for first order spectra) and it's obvious you need rays with pretty small wavelengths to resolve lattices in the order of $0.3\mathrm{nm}$... Pity, really! $\endgroup$
    – Gert
    Aug 27, 2021 at 2:21
  • $\begingroup$ I did this experiment years ago:physics.stackexchange.com/questions/384086/…, which shows it can't only be a case of $d/ \lambda$ ratio. $\endgroup$
    – Gert
    Aug 27, 2021 at 13:31
  • $\begingroup$ And it can't just be about energy either: a VIS laser effortlessly travels through a solid $\text{NaCl}$ crystal. I might raise a ticket myself, intrigued as I am now... $\endgroup$
    – Gert
    Aug 27, 2021 at 13:33

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