Why do x-rays penetrate crystals in diffraction but longer wavelengths reflect?

[photon79]

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 into the interior of the crystal?) why doesn't the reflection occur with big wavelengths?

 

[nbo10]

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]

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.

 

[Pieter Kuiper]

can it penetrate into 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).