Physics 2
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Virtual Activity: The Davisson-Germer Experiment
     The notion that all things have a dual nature (both particle and wave) is a foundational idea in physics. The work of Louis de Broglie gave a theoretical basis for this idea, but it was the famous Davisson-Germer experiment that allowed scientists to exhibit this phenomenon in the lab. Basically, Davisson and Germer fired particles of matter (electrons) at a very smooth and clean sample of nickel. The wavelike nature of the electrons became apparent to them because the angles at which the scattered electrons were most likely found were consistent with the concept of interference from multiple sources, much like a pattern seen from a reflective diffraction grating when using light waves. The virtual activity you are asked to perform here will identify the basic principles of the experiment and allow you to verify two important things:
1. The probability of finding scattered electrons is greatest at specific angles. This verifies the interference of the electrons after reflective scattering. (Use the protractor tool to determine the angles.)
2. The maximum probability peaks are effected by certain parameters and not effected by others.

     Your job is to verify the parameters that affect the angles at which the electrons are most likely scattered. You have several choices in this regard:
a. The incoming speed of the electron. (Velocity slider)
b. The spacing between the atoms in the sample crystal. (Atom separation slider)
c. The size of the atoms in the sample crystal. (Atom size slider)
d. The intensity (number of electrons per second) emitted by the electron gun. (the gray slider under the FIRE button)

     It's very important to know that the pattern produced by the simulation does not model the Bragg equation discussed in the presentation, where the detector is "relatively" far from the crystal (in comparison to the interatomic spacing). Instead the simulation utilizes a mathematical model for a "near-field" pattern. So, one cannot use the equations derived in the lecture with this simulation. However, the effects seen when the parameters change are consistent with those discussed in the lecture.

     Please conduct the investigation as you see fit, taking notes on the changing nature of the pattern of reflective scattering when you change the available parameters. Record all your observations in your lab notebook. The experiment will start by clicking the off/on switch on the electron gun. After you have concluded the investigations, answer the following questions using the theory discussed in the presentation:
1. Does the intensity of the electron beam effect the pattern? Explain your answer.
2. Does the incoming speed of the electron beam effect the pattern? Explain your answer.
3. Does the interatomic spacing found in the crystal effect the pattern? Explain your answer. Also, explain why this experiment can only be conducted with a crystalline solid and cannot be accomplished with an "amorphous" solid whose atoms and molecules are not regularly spaced.
4. Does the atomic size effect the pattern? Explain your answer.

Davisson-Germer: Electron Diffraction
 
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