LecDem Blog

Phun with Electrons: Particle or Wave?

Welcome back to the Physics Demonstrations Blog! We’re back from the winter deep freeze and here to share more fun ideas about physics. This week, a brief look into the quantum world with electron beams!

The electron is a fundamental particle, one of the earliest discovered in modern physics, which has a small but finite rest mass and carries a negative electrical charge.
However, quantum mechanics has taught us that, under certain circumstances, the electron, which we conventionally think of as a particle, can also behave like a wave.

a cathode ray tube with power supply

A stream of electrons traveling through space means a stream of charges is traveling through space – this is, in effect, what an electrical current is. And an electrical current can be deflected by a magnet – so, too, can a beam of electrons. If you hold a magnet near a beam of electrons, like the one in this cathode ray tube, the beam will deflect in a direction perpendicular to the magnetic field.

glowing cathode ray tube, an electron beam is green against a white screenglowing cathode ray tube, green electron beam deflected by magnet

Likewise, if you run two electrical currents parallel to each other, the electric field of each will deflect the other slightly, which can be seen in either two wires running parallel, or by running a wire parallel to the electron beam.two parallel wires being pushed apart by the force of the currents within themglowing cathode ray tube, green electron beam deflected by the current in a parallel wire

This beam is made up of electrons; the glow in the screen behind it is made when a few individual electrons passing by interact with the phosphors in the screen.

a cathode ray tube with power supply, facing end-on towards the viewer

But what happens if we take that beam and pass it through a narrow opening? Conventionally, if you throw a bunch of particles at a hole, either they pass through in a straight line, or they miss and bounce off. But instead, look what happens when we do it with electrons:

cathode ray tube with green electron beam diffracted into ringscloseup of diffraction rings of an electron beam

The electrons are not passing through in a straight line, but instead are diffracting, forming a pattern of rings. This is an interference pattern formed by the interaction of the peaks of a wave. We can see this frequently when light passes through a narrow opening, forming such a pattern.

red laser light forming diffraction rings after passing through a pinhole

But here, it is being formed by electrons – very small particles, but particles nonetheless. Thus, here we see an electron behaving as a wave.