Work sheet: Probabilities in the double-slit experiment

Choose electrons with an energy of 20 keV. Adjust the slit width to 150 nm and the slit separation to 1000 nm. Switch on the source and observe the pattern that appears gradually on the screen. Press the speed button until about 2000 electrons are detected on the screen.

You will obtain a similar pattern as the one shown below.

2. Suppose you were to fire just one more electron through the slits. Can you predict at which position on the screen the electron will be detected? If yes, mark that position in the picture.

Perform the experiment by switching on the source for a short time. A new spot will appear on the screen. Were you able to predict its exact location?
 

3. In the previous experiment you only predicted a single location on the screen. We now modify the experiment a little. Instead of a single electron, let us fire 100 electrons. Can you predict, at which positions you are likely to find many electrons and at which positions only few are found? In the picture below, mark the locations where you think it is likely to find many electrons.

Perform the experiment. To erase the screen click on the screen and press reset. Switch on the source and wait until 100 electrons have been counted. Compare the result with your predictions.

4. Your result was better in activity 3 than in activity 2. Can you explain why? The notion of probability can help you in the explanation.
 

This work sheet can be found in Word97 format under prep\blatt_prep.doc for changing and printing it more easily.

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