Practical Activity for 14-16
Visible diffraction rings on a screen show the wave behaviour of electrons.
Apparatus and Materials
- Electron diffraction tube
- Power supply, 0 - 5 kV (Extra High Tension, EHT)
Health & Safety and Technical Notes
For use with a diffraction tube, the 50 MΩ. safety resistor can be left in the circuit. This will reduce the maximum shock current to less than 0.1 mA..
Leads used MUST have shrouded connectors and insulation capable of withstanding 5 kV.
Make all connections with the power supply turned off. Do not adjust connections while the EHT is switched on.
Electron beam tubes are fragile. Because they are evacuated, they will implode if they break. The tubes are also expensive, so handle them with great care. Use the purpose-designed holders during practical work.
Note that when switching the EHT supply off, it can take a little while for the voltage output to fall to zero. Allow sufficient time before disconnecting.
The electron beam strikes a target of a thin deposit of graphitized carbon on a metal grid situated in the exit aperture of the anode. It is the wave nature of electrons passing through the carbon lattice that produces the diffraction pattern.
The cathode is indirectly heated, so it may take a few moments to warm up.
The wavelength, λ, of the electrons is given by De Broglie’s equation λ=h/ p where h is Planck's constant and p is the momentum of an electron.
- Connect the heater supply of 6.3 volts to the filament.
- Connect the filament to the negative terminal of the EHT supply. Earth this terminal.
- Connect the positive terminal of the EHT supply to the anode of the diffraction tube. Set the accelerating voltage to about 4,500 volts. Carrying out...
- Switch on the heater supply and the accelerating voltage. Rings should appear on the screen.
- Bring a strong magnet close to the tube. Show that the rings are distorted.
- Show how the rings change as the voltage is varied between 3,500 and 5,000 volts.
- The diffraction rings are caused by the electrons diffracting and interfering as they pass through the regular crystal structure of the graphite. They are behaving like waves and the graphite’s crystal structure acts as a grating. As the crystals are arranged at any angle, each crystal produces a diffraction pattern, and their diffraction patterns combine around 360° to form rings. This is the same as the microscope slide made from the cut up pieces of a plastic diffraction grating in the
- Deflecting the pattern with a magnet shows that the pattern is being produced by moving charged particles, rather than light or some other form of radiation. It introduces the dual nature of the electrons: they behave like particles when they are accelerated, like waves as they pass through the graphite foil, and like particles again as they are deflected in the magnetic field. This is an example of complementarity. See guidance note...
- The diameter of the rings will change as the accelerating voltage changes. It seems that the more energy the electrons have, the shorter their wavelength.
- The pattern resulting from the passing of a beam of electrons through graphite is very similar to a beam of X-rays passing through the powdered potassium alum crystal. It suggests that electrons undergo diffraction, and will therefore interfere in the same way as X-rays and other waves.
This experiment was safety-tested in March 2008
A video showing how to use an electron diffraction tube: