Physics Photo of the Week
Cathode ray tubes and electrons striking the phosphorescent
inside the tube are nothing new. They have been used for years
for television displays before the days of LCD screens. What you
don't normally see on TV are the diffraction effects of electrons which
are a result of fact that electrons - normally thought as particles -
behave as waves that can demonstrate many of the phenomena of wave
behavior. Diffraction is a property of waves. Therefore
these diffraction patterns demonstrate that
electrons consist of waves. In this case the diffraction is
caused by the electrons passing through a thin layer of polycrystalline
graphite (pencil "lead"). The graphite crystals can be modeled as
hexagonal arrays of a "chicken-wire" lattice. The regular array
of carbon atoms in the crystals is responsible for the diffraction
green glow is caused by electrons in a cathode ray tube striking the
phosphorescent coating on the inside of the glass bulb just behind the
ruler. In addition to the bright spot where the beam of electrons
is striking the coating in the bulb, one can see two concentric rings
indicating where more electrons strike the coating than the general
by Robin Dhakal, Physics II class
The wave nature of electrons was first proposed by Louis deBroglie in
1924. Students of physics learn the deBroglie hypothesis
relating the wavelength λ
of electrons to the momentum p
DeBroglie's bold hypothesis was soon proven to be true by an
experiment by Clinton Davisson at Bell Telephone Laboratories in 1927
and by George Thompson at the University of Aberdeen in Scotland.
The seminal works were truly deemed important. DeBroglie was
awarded the Nobel Prize in Physics in 1929 for predicting the wave
nature of electrons. The experimental proof of the wave nature of
electrons by means of these diffraction experiments by Davisson and
Thompson later earned the 1937 Nobel
Prize in Physics.
Why should we care that electrons are waves rather than
particles? For one thing we could not exist nor could atoms of
matter exist were it not for the wave nature of electrons discovered
about 80 years ago. An atom of matter consists of a tiny,
extremely dense nucleus in the center. Electrons circulate around
the nucleus. However, charged discrete particles such as
electrons orbiting a nucleus are not stable in classical physics.
A charged particle in a tiny orbit will gradually radiate energy as a
vibrating electric field. This vibrating electric field will
generate light waves and conduct energy away from the atom. Cell
and radio transmitters radiate radio waves in a similar manner
due to electrons vibrating up and down an antenna. The problem of
stable electron orbits in atoms puzzled physicists for many
until the advent of quantum mechanics about 80 years ago. Quantum
mechanics assume that electrons are waves. An electron's "orbit"
around a nucleus is best thought of as a standing wave rather than a
discrete particle in orbit. A
simple model of an orbital wave is a loop of stiff wire that is set to
vibrate. At certain frequencies of vibration, a circular standing
wave can be seen on the vibrating wire as shown in the drawing at
right. The electron wave in an
atom consists of a similar standing wave. The electron wave in an
atom is 3-dimensional, however, and the mathematics is quite
complicated and the topic of quantum mechanics. However, we owe
our existence to the existence of stable atoms of carbon, hydrogen,
oxygen etc. and the wave-nature of the electronic orbits in these atoms.
For another discussion of electron diffraction, see the PPOW
for November 4, 2005.
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.
All photos and discussions are copyright by Donald
Collins or by the person credited for the photo and/or
discussion. These photos and discussions may be used for private
individual use or educational use. Any commercial use without
written permission of the photoprovider is forbidden.
here to see the Physics Photo
the Week Archive.
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