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Take any macroscopic crystal of VIS transparent monocrystalline type of material like table salt ($\text{NaCl}$), sugar (sucrose), quartz (silica), calcite ($\text{CaCO}_3$), glass (which is non-crystallin, of course) etc etc. A VIS laser beam travels through these crystals effortlessly.

But grind the same materials to a fine powder and they become thoroughly opaque to VIS.

Now replace the laser with a collimated X-ray beam and the powders are transparent to it, producing a diffraction pattern (if the set-up is correct)

So why are the powders transparent to X-rays but not to VIS?


This question is inspired by this Q and A.

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The reflectivity, in the visible range, of your samples is nonzero. For large crystals, the light (generally) only passes thru two interfaces, i.e. air-crystal and crystal -air , and thus rather little signal is lost.
WHen you grind it up, not only do the crystal faces of each particle point in random directions, but the light passes thru many many particles, getting both attenuated and redirected (prism) along the way.

So far as visible vs. X-ray: the wavelengths are rather different, so at the atomic level very different things happen. Roughly speaking, the X-rays see the crystal lattice as a diffraction grid. That same grid spacing is far too small to diffract visible wavelengths.

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