Home >> Quantum, electron diffraction
What is Diffraction?
Classical physics gives us a definition of diffraction in terms of waves:
Diffraction is the phenomenon whereby light or sound waves bend around small
obstacles or spread out through small openings.
The Diffraction Grating
A diffraction grating is an array of identical, equidistant, parallel lines on a surface.
Gratings are used to produce optical spectra from a single source, parallel beam of light.
There are two types of grating: reflection & transmission
The Transmission Grating:
Light rays are diffracted at the edge of each gap in the grating.
The waves of light add when the path difference is a whole number of wavelengths.
So discrete images at different angles are produced.
'n' values correspond to:
n=0 no path difference
n=1 one wavelength path difference
n=2 two wavelengths path difference etc.
Bragg Diffraction (an example of reflection diffraction)
Bragg diffraction occurs when particles or waves with wavelength 'λ' comparable with atomic spacings 'd' , interact with atoms in crystals.
At a particular angle θ* waves are diffracted by atoms in adjacent planes and interfere constructively.
As with transmission diffraction gratings, a bright image is only produced when the path difference is a whole number of wavelengths.
* the angle the incident wave makes with a plane of atoms
Practically, the incident waves can be provided by X-rays, electrons or neutrons.
Electron Diffraction Interference Patterns
Electron, X-ray and Neutron Diffraction Compared
The most commonly used method is X-ray diffraction.
|scattering||by electrostatic repulsion of nucleus||by electron cloud around nucleus||by interaction with the nucleus|
|penetrating power||poor (requires thin specimens)||good||good|
|matter interaction||high (unreliable results)||none||moderate|
|magnetic effects||no||no||yes (neutrons have their own magnetic field)|
|good for light elements||no||no||yes|
|particular uses||crystals||crystals||fuel rods, archaeological artefacts|
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