Physics Photo of the Week

May 9, 2008

Electron Diffraction
This eerie 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 surrounding area.
Photo by Robin Dhakal, Physics II class
Cathode ray tubes and electrons striking the phosphorescent coatings 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 effects. 

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 by Planck's constant h:
λ = h/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 phones 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 years 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.
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 dcollins@warren-wilson.edu. 

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.


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