PHYSICS 320 LABORATORY
THE DIFFRACTION OF ELECTRONS
The purpose of this experiment is to demonstrate that electrons have wave properties. Specifically, we wish to show
The de Broglie wavelength of a material particle is
where h is Planck's constant. For electrons accelerated through a potential difference V, the velocity v can be obtained from the classical expression
and substituted into the de Broglie relation obtaining:
The Bragg condition for diffraction for small angles is
where d = the interatomic spacings, D is the ring diameter, and L is the path length from the carbon target at the gun aperture to the luminescent screen. Combining this with the previous relation,
D and V are the only variables, so the relation can be verified by means of a graph related to these variables.
The electron diffraction tube, TEL.555, comprises a 'gun' which emits a narrow converging beam of electrons within an evacuated clear glass bulb on the surface of which is deposited a luminescent screen. Across the exit aperture of the 'gun' lies a micromesh nickel grid onto which has been vaporized a thin layer of graphitized carbon. The beam penetrates through this carbon target to become diffracted into two rings.
Connect the tube as shown in the following diagram:
Filament Voltage ( VF ). . . 6.3 VAC
( 8.0 V max ) from TEL 2813
Anode Voltage ( VA ). . . . 2500 - 5000 VDC from TEL 2813
External Bias ( VB ). . . . . . 0 - 50 VDC from TEL 801
Anode Current Meter ( IA ). . . 0.15 mA at 4000 V ( 0.20 mA max.)
Carbon atoms in a graphite structure are arranged in a hexagonal rather than cubic manner. Using the geometry in the above figure, what is the ratio of the d values? Compare this theoretical value to your experimental results. The bond length for carbon graphite is 0.142nm. Calculate the bond length from your results and compare to the accepted value.