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 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.
Click
here
to see the Physics Photo
of
the Week Archive.
Observers are invited to submit
digital photos to:
