The Modern Physics Puzzler

The diagram below shows the classical particle behavior for a uniform beam of electrons striking a barrier with a narrow slit. The electrons that pass through the slit travel to the screen, where they are registered. The resulting pattern on the screen is simply the shape of the slit itself.

The second diagram below shows the quantum mechanical behavior for the beam of electrons behaving as waves. A single slit diffraction pattern, similar to that produced by light, forms on the screen.

• In each case, how does the width of the slit compare to the deBroglie wavelength of the electrons?
• Consider the second case. Using the Uncertainty Principle, explain how an electron can pass through the slit, yet arrive at the screen outside of the classical "shape of the slit" region. Be specific in your arguments concerning uncertainty in position and momentum.

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