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Physics Photo of the Week

November 4, 2005

Electron diffraction
Electrons are fascinating "particles".  They behave both as waves and as particles.  The electrons that orbit the nuclei of atoms are the same things that travel in wires, cathode ray tubes (CRTs), and transistors.  This is a photograph of the face of a special CRT.  The electrons travel from the back of  the tube toward the front which has a coating to cause the green glow.  Between the source of the electron beam and the screen is a thin layer of polycrystalline graphite.  The graphite has an atomic structure like "chicken wire" - the carbon atoms in the graphite crystals all lie on the vertices of hexagons.  See the picture below.
Figure 1.  The front face of the electron diffraction apparatus.  The black tape is placed over the bright central spot to avoid overexposing the camera.  Photograph taken by Physics II class in spring, 2005.

When electrons strike the graphite lattice, the electrons are scattered in various directions.  However, because electrons are not point-like particles, but consist of waves with a particular wavelength, the waves interfere with each other as they emerge from different openings in the lattice.  This interference is called diffraction.  Diffraction of electrons is therefore proof that electrons have wavelike properties.

Figure 2. (Left): The hexagonal lattice of graphite consists of hexagons resembling "chicken wire".  (Right): The schematic cross section of the electron diffraction tube.

A major consequence of the wavelike properties of electrons is the fact that electrons in atoms are not point particles in orbit about the nucleus.  Since the electrons are wave-like, the electrons in orbit about the nucleus are circular or spherical standing waves.  Since the circumference of the electron's orbit must be an integer number of wavelengths, only particular wavelengths of electrons will fit.  This is the essence of quantum mechanics: electrons exist in atoms in discrete states, not any state.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature an interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation.  Atmospheric phenomena are especially welcome.  Please send any photos to

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.