Physics Photo of the Week

Warren Wilson College

March 11, 2005

Magnetic Deflection of Electrons

Photo by Physics II Class
This is a picture of a special cathode ray tube (CRT) - very similar to a television picture tube and works on the same principles.  In this special tube, electrons originate from the right hand side of the picture and are travelling to the left in a vacuum.  The electrons, however, strike the graph paper (called a "graticle").   Each major division is one centimeter.  The graticle is coated with a "phosphor" that glows when electrons of high eneough energy strike it.  This phosphor glows blue when the electrons strike it.  A color TV has three different types of phosphors - those that glow red, green, and blue in order to make a color display.  In the picture above, the beam is bent.  This bending of the electrons' path is caused by the magnetic force provided by an electro-magnet surrounding the tube.  The magnetic field from the coils is directed so that the field is directed coming out toward the camera.  If a magnetic compass were placed in front of this tube, the compass points towards the observer.  The forces that magnetic fields exert on charges are unusual in that they are not like ordinary non-magnetic forces.  Magnetic forces are perpendicular to both the magnetic field direction and the velocity of the charged particles.  The electrons (charged particles) move basically from right to left in the above photo.  The magnetic field is perpendicular to the plane of the photo and perpendicular to the velocity of the electrons.  The magnetic force is perpendicular to both the velocity and the magnetic field, so the force is down.  The electrons follow a circular path because the force is always at right angles to the velocity.  By analyzing and measuring the radius of the electron's orbit, knowing the velocity of the electrons, the physical properties of the electrons, (the ratio of charge to mass) can be calculated.  This experiment was pioneered by Nobel laureate J. J. Thompson in 1897 trying to learn about the mysterious cathode rays in evacuated tubes.  This contributed to the discovery of the electron., and of course, the demise of cathode ray tubes for the purpose of TV.  Read an interesting biography on J. J. Thompson at the American Institute of Physics website.

The animated picture above shows the effect of different magnetic fields on the deflection of the electron beam.  The stronger magnetic field causes the beam to be bent more because the magnetic force is proportional to the strength of the magnetic field.

If you look carefully at the images above, you will see the physics classroom and the people in the lab.  This is a reflected image off the spherical glass envelope surround the tube.

This is the last Physics Photo of the Week until March 25 due to spring break at Warren Wilson College.

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

Click here to see all Physics Photo of the Week for 2005

Click here to see all Physics Photo of the Week for 2004.