What is true about d in bragg's law? please explain.
1- The d is distance of bond (covalent or electrostatic) between two atoms which are located in adjacent bragg's planes and is in angstrom range. and is the reason of usage of x-ray wave for crystallography (in spit of gamma or Uv...)
2- The d is distance of two corresponding atoms of two molecule in a unit cell or in two adjacent unit cells which are located in adjacent bragg's planes. and is no the reason of usage of x-ray.
I want to know that we use x-ray wave because the d is in angstrom range? or we use x-ray because its wavelength is in range of atom's radius and atomic bonds which could be seen by x-ray?or both?.
The d is distance between two corresponding atoms (ions, molecules) in the crystal structure of the solid phase which are located in adjacent bragg's planes. It is in angstroem range. So, we can use the x-ray wave for determination of lattice constants of the solid phase.
The simple answer for your question is:
The wave length of X-ray is in the range of distance of interatomic planes dhkl within a unit cell of crystals.
both 1 and 2 statements are reasons for using X-ray. You should think of the X-ray wavelength as the best you can do in resolving two objects so that wavelength 1 Angs means any two atoms separated by that distance is the best you can do. Where the d spacing comes in is that you have all the possible spacing from low resolution with spacing of around 100 Angs all the way to 1 Angs, so that each d spacing is looking at different molecular spacings. Remember lamda=2d sin(theta) so that every different angle is like changing the wavelength so that d becomes larger as angle is smaller and therefore can resolve only objects separated by larger distances. But this is important in providing contrast to the molecule so you can distinguish it from the background
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