Interference

For a detailed lesson template with learning outcomes, keywords etc. click here.

This lesson was developed before the Perimeter Institute of Theoretical Physics developed their wonderful 25-minute video and Teachers Guide on the same subject called The Challenge of Quantum Reality. You should order your free copy today and use it instead of or in addition to the lesson here.

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Left: PhET Computer Simulation, Middle: Electron Intereference, Right: Buckyball Interference
Images are courtesy of PhET, Hitachi and A. Zeilinger

Resources

0) Experiments in Classical Interference with Light (60 minutes)
Before you do quantum interference, your students need a firm grounding in classical intereference. Take a look at these diffraction experiments with light . Students use laser pointers to measure tiny objects and there is a brilliant diffraction demonstration using pencils and elastics. These experiments are from the Centre for Nanoscale Systems Insitute for Physics Teachers at Cornell University. I haven't had time to review the other offerings yet, but will soon.

This lesson involves lasers which have safety concerns that you should be aware of.

1) Interactive Lecture using a Computer Simulation (45 minutes)
The PhET Quantum Wave Intereference simulation is used to explore the behaviour of electrons as they pass through one and two slits. Show the 5-minute video of real electron interference from Hitachi (middle image) and real experimental data that demonstrated the interference of Buckyballs made of sixty carbon molecues (right image)! The students need a copy of the double-slit worksheet. Details of the lesson and the answers to the worksheet can be found in the teacher's notes.

2)Whiteboarding Excercise with a video (15 minutes)
Show the students the 5-minute animation of Dr. Quantum. This is a clip from the movie "What the Bleep?" which purports to be about physics but is in fact a lot of really flakey philosophy, hitching a ride on the ideas of quantum physics. It is really well animated but is full of very bad physics. Have the students find all the physics errors that they can - there are lots of them! 1) The water changes wavelength after going through the slits. 2) There isn't enough diffraction spread from the slits to provide the overlap needed for intereference. 3) The two-slit pattern shown, is actually a single-slit interference pattern. 4) The top view of the water wave interference doesn't seem to match the pattern that shows up on the screen.

3) Homework (30 minutes)
The last three questions in the worksheet have the students look at the three resources (simulation, video, experiment) again on their own in more detail.

For more about the PhET website and simulations, click here.

4) Optional Video (78 minutes or 20 minute section)


Richard Feynman (see picture above), is the physicist that popularized the use of the two-slit experiment to teach almost all of the key aspects of quantum mechanics. (It is a little weak on entanglement and the discrete nature of quantum states. Polarized light is better for those.) He didn't stop with two-slit interference. All the wave phenomena and their wave explanations must be fundamentally due to quantum physics - all you have to do is turn the intensity down until there is only one photon at a time.

He did a public lecture in New Zealand about quantum physics and used thin film interference as his vehicle. You can watch it here from the Vega Science Trust. (You can also buy a copy from physics@auckland.ac.nz.) It's well worth watching the whole thing yourself, but I would not show it all to a class. The section from 40 to 60 minutes are the best. From 40 to 49 minutes he points out how partial reflection and refraction doesn't make sense if light is a particle. From 49 to 55 he looks specifically at thin film interference and how that doesn't make sense if light is a particle. From 55 to 60, he gives a very simple, visual understanding of the 'math' involved in computing the probability of reflection and emphasizes that this math works but that there is no explanation of why it works.

5) Optional Video (12 to 60 minutes)

This is a public lecture that Akira Tonomura gave in 1994. You can view it here from the Vega Trust. For the first 20 minutes he does a nice review with demonstrations of magnetic field lines and wave interference - 1-D and 2-D. From 20 to 32 minutes he describes how he did the double-slit experiment with electrons. This part is the most relevant to this lesson. The rest of the film looks at his work using electron holography to examine superconductivity and results in movies showing tiny magnetic field vortices in one diection annihilating those spinning the other way.

Last Updated January 2009