Physics Photo of the Week
November 4, 2005
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.
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
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
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 firstname.lastname@example.org.
here to see all Physics Photo of the Week for 2005
to see all Physics Photo of the Week for 2004.