# Some basic x-ray questions!

In x-ray diffraction with crystals, what actually happens with the incident x-ray beem or photon? scattering occurs due to which atoms (plane) of the crystal?(can it penetrate in to the interior of the crystal?) why doesnt the reflection occur with big wavelengths?

All the planes diffract the x-rays to some extent. The peaks you see in diffraction data can be index to specific planes, ie 100, 110, ...Keyword lookup: Structure factor. Of course you could use longer wavelengths but the spacing between the planes would have to be larger.

Gokul43201
Staff Emeritus
Gold Member
The largest plane-spacing will be the largest lattice parameter. All other planes are closer together. So, $d \leq a$. Also, $sin \theta < 1$ and $n \geq 1$ .

Hence, $$\lambda = \frac {2dsin \theta}{n} \leq 2a$$

So, no wavelength greater than twice the largest lattice parameter can produce a diffraction peak.

photon79 said:
can it penetrate in to the interior of the crystal?
X-rays penetrate humans; they also penetrate crystals. A single plane of atoms absorbs very little. A single plane also reflects very little. Only when hundreds or thousands of planes contribute to the reflection, you can get sharp Bragg diffraction at certain angles (when phases add constructively).