Electron Diffraction

We have seen how light can behave like discrete particles, now we shall see an experiment in which electrons show wave properties. The first person to do this was G P Thomson, the son of J J Thomson, the person who discovered the electron.

J J Thomson	G P Thomson	de Broglie

The target is a metal grid which has been sprayed with graphite. The rows carbon atoms in the graphite act as a diffraction grating.

Electrons are fired at the target and a diffraction pattern is seen on the screen of the evacuated tube. Pairs of rings are seen. This is because there are 2 distinct atomic separations in graphite (see above), each one acting as a separate value of d in the equation n = d sin Ø. 
The large ring corresponds to the smaller spacing etc..

This is a very important experiment. We see electrons diffracting. They are passing through slits and producing an interference pattern. This is evidence that particles can behave like waves.

Something else interesting happens when we increase the accelerating voltage V. We find that the pattern gets smaller, the electrons have a smaller wavelength when they are traveling faster.

The de Broglie relationship expresses the relationship between the momentum of a particle and its wavelength.

So do all particles behave like waves? 
Would a cricket ball have a wavelength? 
If lots of cricket balls went through a gap would we see a diffraction pattern?

There would be a diffraction pattern but we wouldn't see it. The de Broglie wavelength of a typical cricket ball is VERY small and so the pattern seen wouldn't be very spread out, impossible to detect in fact.

Only tiny little particles like electrons (neutrons, protons too) behave like waves, sometimes.