Electron Interference

In this activity we will use a simulation program to observe photon and electron interference. We suggest that students also have an opportunity to see and use equipment such as lasers, diffraction gratings and electron diffraction tubes if available. An experiment with real light should, preferably, precede this activity.

Instructions:
1) Open the Diffraction Suite program and select Double Slit from the menu.
2) Click on the source tube labeled Photons fourth from the left side of the screen.
3) Move the Slit Separation slider to approximately 10000 nm.
4) Click Start and let the program run until enough photons are present to form a pattern.

The pattern that accumulates should resemble the double slit interference pattern we observe for light from a laser (areas of bright and dark).

To refresh your knowledge of light interference repeat the process for varying wavelengths of photons. You can save and compare your runs by clicking and dragging the pattern over to the spaces provided on the right hand side of the screen.

Once you have experimented with the program answer the following questions:

• How does the interference pattern change with wavelength?
• Given that Energy is inversely proportional to the wavelength, how does the interference pattern change with light energy?

Select the source tube labeled Electrons on the left side of the screen. Click Start to see what happens. Sketch the pattern that accumulates.

In comparison with the photon interference patterns you have observed answer the following questions:

• Which pattern(s) - light, waves or particles - does the pattern for electrons resemble?
• What can you conclude about the behavior of electrons in a double slit experiment?

Similar experiments resulted in a conclusion that electrons behave as waves. This result was surprising but inescapable. As we shall see, it is confirmed in many situations.

Use the simulation program to determine how the electron interference pattern changes with the energy of the electron. Try four different electron energies. Record your observations.

Recall how the patterns for photons (or light) changed with wavelength. Let us suppose that an electron has a wavelength related to its energy in the same way as light. Using this information, describe below how the electron's wavelength changes as the energy increases.

The simulations we did with electrons seem to imply that wave behavior is associated with electrons. As electron energy increases the distance between dark regions in the interference pattern decreases. If we compare this to our observations of light we could say that as the electron energy increases the electron wavelength decreases.

Let us now examine some other small particles and see if they exhibit wave behavior.