Homework lab

Lab 12: Electron Diffraction
PHYS 242

ã 2003-5, University of Maine Physics Education Research Laboratory. Portions of the materials in course pretests,
tutorials, and homework were developed by the Physics Education Group at the University of Washington and the
Physics Education Research Group at the University of Maryland. (materials further adapted at CSU)

In today’s activity, we will consider ways in which a single object can do two contradictory
things. In order to do this, we first need to be sure that we know what we know!

I. Thinking about everyday examples of things
1. Imagine shooting a paint ball at a wall (for those who don’t know, a paint ball is like a

little balloon filled with paint). What would you see? What would you see after
shooting 10 paintballs? 100?

2. Imagine randomly shooting 100 paint balls at a wall with two holes in it. What would
happen? What would we see beyond the wall if we had a screen there?

3. Imagine a water wave coming up on a wall with two slits in it. What happens? (Hint:
remember our interference lab)

II. A reminder from last time

It’s important for us to remember ideas from the last activity because we need to keep them in
mind while doing new things. We’ll just remind you of a few things from before, and then
move on.

Recall the experiment when you set up a laser and directed the laser at a screen.

1. What happens when you shine a laser pointer at a blank screen with no slits? Draw a
sketch.

Lab 12: Electron Diffraction PHYS 242

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B. A portion of the pattern produced by a laser beam passing through two very narrow slits

has been reproduced below:

Note the parts of the picture which are bright and which are dark and recall how we
modeled the existence of (1) dark lines and (2) light shining on parts of the screen at which
the laser isn’t pointed.

C. Suppose the left slit were covered. Sketch what you expect to see on the screen. (Hint: This

is now a narrow slit).

Check your answers with your instructor.

III. Photons

Another name for a laser is a photon gun. A photon is a small particle of light, and a photon
gun like the ones we use in the classroom usually shoots out a very large number of photons
each second. The number of photons that are emitted at a time is related to the intensity:
more photons mean more intensity, which means the laser looks brighter.

Photons, No Barrier

A. Suppose that the intensity of light that the laser puts out is lowered so that it only emits one

photon at a time. Suppose the laser shines at a photographic film directly (without a barrier
in the way). A film turns color when a photon hits it, and then stays that color.

1. The big circle represents a normal laser dot (the area in which photons can hit the screen).
Sketch what you would see on the plate after ten photons hit in the circle at the right. How is
this like what happens with paint balls?

center

Lab 12: Electron Diffraction PHYS 242

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2. On the second circle, mark what you would see after a very long time.
Explain your reasoning. Is this consistent with the idea of lots of paint
balls hitting a wall?

Photons, Double-Slit

B. The laser light in part A now shines through two slits.

The film is located behind the slits. After a short time, a
portion of the resulting pattern on the film is shown at
right.

1. Is the pattern you observe consistent with the water waves we have studied? Why or
why not? Be explicit about the features of the pattern that guide your reasoning.

2. How is the pattern you observe consistent with your description of paint balls on page
1?

Check your answers with your instructor before continuing.

IV. Electrons
A. An electron is like a very, very small paint ball. Without the paint. Like paint balls from

paint ball guns, we can shoot electrons from electron guns. Your television has an electron
gun in the back. It shoots electrons at the screen, which glows when an electron hits it.
Suppose that an electron gun shoots out one electron at a time, and those electrons hit a
photographic plate. A photographic plate is like film, and it lets us
see where electrons hit.

1. The circle on the right represents the plate. Sketch what you
would see on the plate after ten electrons hit it. How is this
like what happens with paint balls?

center

Lab 12: Electron Diffraction PHYS 242

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2. On the second circle, mark what you would see after a very
long time. Explain your reasoning. Is this consistent with the
idea of lots of paint balls hitting a wall?

A Few Electrons, No Barrier

B. Now let’s test our predictions. Go to the following URL:

https://phet.colorado.edu/en/simulation/legacy/quantum-wave-interference

**This simulation requires java and is not compatible with iPads**

• Start the simulation by clicking “play”

• Next select the “Single Particles” tab at the top of the page. In the drop-down box at

the bottom of the page, select “Electrons

Lab 12: Electron Diffraction PHYS 242

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• In the “Gun Controls” box under the drop-down box, check the box for “Auto-
Repeat.

• Now, click “Fire” on the Electron Gun to begin firing electrons at the photographic
plate. After about 10 electrons have hit, pause the simulation.

1. Describe or draw what you see on the photographic plate.

2. How does this compare with your prediction from Section IV.A.1 (Electrons with no
barrier)?

Electron
Gun

Drop-down
box

Photographic
Plate

Lab 12: Electron Diffraction PHYS 242

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Many Electrons With No Barrier

C. Now let’s test our prediction for what we would see after shooting electrons at the screen

for a long time. Instead of shooting single electrons and waiting a long time, we’ll just shoot
a whole bunch of electrons at the screen all at once.

• Switch over to the “High Intensity” tab of the simulation

• Select “Electrons” in the drop-down box.

• Click the “On” button on the Electron Gun

• Wait a few seconds and then pause the simulation

1. Draw or describe what you see on the photographic plate.

2. How does this compare with your prediction from Section IV.A.2 (electrons with no
barrier)?

A Few Electrons, Double-Slit

D. Now the beam of electrons is directed at two slits. A photographic plate is located behind

the slits.

1. Predict what you will see on the photographic plate for the double slit experiment after
a few electrons are shot at the screen. Draw or describe your prediction here. (Hint:
How does it compare to what you would see with photons?)

Let’s simulate this using the PHET simulator again.

• Switch back to the “Single Particles” tab.

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• In the sidebar, click on “Double Slits >>”

• Turn the “Slit Width” slider down to the minimum

• In the drop-down box, select “Electrons”

• In the Gun Controls box, select “Auto-Repeat”

• Now, click “Fire” on the Electron Gun to begin firing electrons at the screen. After

about 10 electrons have hit, pause the simulation.

2. Do you see a pattern in the dots created by the electrons? Draw or describe what you
see.

3. Three times now, we’ve said that electrons are like paint balls. In this scenario, we’re
shooting electrons at a 2-slit mask, just like we did with paint balls in I.2. Does this
picture look similar to what you predicted in I.2.?

V. Reinterpreting Electrons OR Electrons are not really paint balls.

The experiment of electrons being shot at the two-slit mask continues …

Many Electrons, Double-slit

A. Let’s see what happens after a long time passes. Once again, to simulate this, we will

increase the intensity of the electrons that we are firing at the screen.

• Switch back to the “High Intensity” tab in the simulator

• In the sidebar, click on “Double Slits >>”

• Turn the “Slit Width” slider down to the minimum

• In the drop-down box, select “Electrons”

• Turn the electron gun on and let it run for a few seconds.

1. Draw or describe what you see on the photographic plate.

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2. How is this picture similar to the sketch you made for a handful of electrons with no
barrier (section IV.A)?

3. How is it dissimilar?

4. Do you think the black lines will ever fill in? Why or why not?

5. How do your observations compare with this image of electron diffraction?

C. We can use our observations to figure out certain properties of electrons.

1. How, if at all, is it appropriate to say that electrons behave like waves? What must we
observe to conclude that a thing is like a wave?

2. How, if at all, is it appropriate to say that electrons behave like particles?

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VI. Finding consistencies

In this lab, we’ve studied paint balls, electrons, and photons, and we’ve reminded you of water
waves. The questions in this section are a place for you to summarize the ideas developed in
this lab.

A. In what ways do electrons behave like paint balls? Support your answer by description and

with reference to specific experiments.

B. In what ways do electrons behave like water? Support your answer by description and with

reference to specific experiments.

Check in with your instructor before moving on to the post-lab knowledge check!

Electrons and photons do amazing things. They can act like waves and they can act like particles. The
observations we have made in this activity serve as a basis for much of the field of quantum mechanics.