Diffraction of x-rays by crystals

[sarah786]

:shy: By looking at the figures showing diffration of x-rays by crystals, it looks more like reflection.. Seems like I am studying reflection... So, where is the diffraction part??

 

[Moorebc]

The geometry is similar to reflection, and most x-ray diffraction is actually done with the incident and diffraction angles equal. However, the path difference criterion means that constructive interference only occurs at a finite number of angles for a specific interatomic layer separation, only one for a given order. Reflection is, of course, a continuous function.

 

[sarah786]

thanks for the effort but honestly, i didn't understand your answer... I was asking that it seems more like reflection...in diffraction, the light ray passes through the slit and bends.. but by looking at the diagram I perceived that the x-ray was just bouncing back...

 

[sophiecentaur]

Sarah, are you confusing diffraction with refraction? Only you use the word "bends".

You only get Xrays scattered in certain directions, when the angle between the incident ray is appropriate for the spacing of the planes of atoms in the lattice and constructive interference occurs. It's called reflection because, when it happens, the incident and reflection angles are the same. (Wow. I just checked this in a section of a textbook that I last used in 1967 for my degree!)
This 'reflection is nothing to do with reflection off the surface of the sample but refers to the planes of atoms that can be identified within the structure.
Actually interpreting these scattered 'reflections' and relating their angles to lattice spacings is pretty hard because it's all in 3D.

A much simpler example of this sort of diffraction pattern would be what you get from an optical diffraction grating, where the angles of the scattered rays relate to the wavelength and the line spacing.

 

[Moorebc]

thanks for the effort but honestly, i didn't understand your answer... I was asking that it seems more like reflection...in diffraction, the light ray passes through the slit and bends.. but by looking at the diagram I perceived that the x-ray was just bouncing back...

The two beams passing through the slits add up or cancel out at certain angles because there is a path length difference between them. That produces a fan of alternating light or dark interference fringes. It would work the same if there were a whole series of slits side by side.

In x-ray diffraction from a crystal, the x-rays are bouncing back from different layers of atoms spaced particular distances apart. The reflection from each layer interferes with the ones from the layers above and below in the same way as the beams through the two or more slits - giving the diffraction pattern. The only difference is that the slits are acting in transmission while the x-rays are in reflection

http://pubs.usgs.gov/of/2001/of01-041/htmldocs/images/beam.jpg"

Incident ray 1 reflects from point A. Ray 2 travels an extra distance B-C-D so scattered rays 1' and 2" are out of step and will reinforce or cancel out depending on how that distance compares with the wavelength.

 

[michel99]

I want to know "What does mean by pseudotranslational symmetry ?"