Home >> Quantum, electron diffraction

What is diffraction?

Classical physics gives us a definition of diffraction in terms of waves. It is the phenomenon whereby light or sound waves bend around small obstacles or spread out through small openings.

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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.

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Bragg Diffraction (an example of relection 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.

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Electron diffraction interference patterns

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Electron, X-ray and neutron diffraction compared

The most commonly used method is X-ray diffraction.

Summary:

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