The de Droglie wavelength 

In 1923 Louis de Broglie asserted, by way of a somewhat obscure argument, that what were thought of as material particles of rest mass m might in fact have an associated wavelength given by....

... where p is the linear momentum mv.

The remarkable happened. The electron, proton etc. were soon found to have wavelengths as de Broglie had suggested!

Electron diffraction patterns

Two rings, due to the two cubic grid spacings in a graphite sheet. The center of the pattern was overexposed and has been masked.

Increasing the anode voltage (electron energy) reduces the size of the pattern.

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