Physics Photo of the Week

April 29, 2011

Diffraction of Electrons - Photo by Amelia Hubbard
This is an image of the glowing electron tube showing that electrons are wave-like particles.  In this device a beam of electrons is directed toward the coating on the inside of the glass and causes the coating to glow.  It is a forerunner of an old-fashioned TV picture tube, except that the image is much  more interesting than what is played on televisions!  A photo of the whole tube is shown below.  The diffraction pattern (two concentric circles) shows that the electron wavelength is only about 10-11 meters (about 1/10 the size of atoms).

A beam of electrons is directed toward the viewer.  Most of the electrons strike the front of the tube in the center of the screen and produce a very bright dot.  The bright dot is covered with a rod to prevent overexposure to the camera.  A transparent ruler (cm scale) is placed over the front of the screen to permit easy measurement of the pattern.  Between the source of the electrons and the screen is a very thin target of a thin film of powdered graphite - the same type of graphite that is used for pencil "leads".  The atomic structure of graphite - consisting of an array of hexagons like chicken wire - acts as a diffraction grating.  An ordinary optical diffraction grating deflects light from its straight path due to the fact that light is a wave.  The graphite in this instrument acts to deflect the beam of electrons because the electrons are characterized by a wave length. 
This instrument proves that electrons are characterized as waves.

The wave nature of electrons is fundamental to our understanding of matter and the structure of atoms.  The electrons in atoms exhibit discrete energy levels - all due to the wave nature of electrons.  See PPOW for April 16, 2004 for discrete atomic spectra.  Without the wave nature of electrons, matter as we know it would be entirely different.  There would probably be no life.

Other discussions of this experiment can be seen on PPOW for November 4, 2005 and May 9, 2008.

Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department.  These photos feature 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 

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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