Electron Diffraction

Light is a wave, right? No, said Einstein, when he discovered the photoelectric effect, it’s a particle. A light-bullet, or a photon with  energy = h x frequency which is equal to hc/λ. Actually, it’s both.

But, we’ve been here before.

Electrons are particles, right? No, they’re waves, fitting around a tramline round a nucleus, the length of which is arranged so that the electron fits nicely as a standing wave around the orbital. Have a look at this little animation and try to fit a few waves around a circle.

..think of two-dimensional diffractionThe image is an electron diffraction photograph of a  crystalline structure like salt which indicates that the wavelengths must be very small indeed since the apertures are of the order of interatomic distances. Electrons don’t penetrate matter very well, so this is a thin film crystal – in two dimensions, more or less.

Confusing, isn’t it… Some properties are best explained using a particle model, others by a wave model.

If electrons are wavelike, they should have a wavelength and thus should be able to be diffracted.  Wavelength first. Count Louis de Broglie (pronounced ‘de Broy’) came up with this in 1929, ten years after Bohr’s model of the atom..

wavelength = h/momentum

λ=h/p = h/(mv)

h being Planck’s constant. In short, everything with mass that moves has a wavelength.

I wonder what the wavelength of a bowling ball might be? From a high school factbook, we learn that the maximum permissible mass of a bowling ball is 7.26kg. Assuming it travels at about 10m/s when hurled down the lane, its wavelength would be nine times ten to the minus thirty six metres. A bit too small to measure, then…

So, this is only important if masses are very small.


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