Electron Double-Slit Interference

1 · Predict

Electrons fired one at a time through two narrow slits build up an interference pattern on a screen, just like light waves do. Does a faster electron (shorter de Broglie wavelength) produce wider or narrower interference fringes?

• Narrower fringes — fringe spacing is proportional to wavelength, and faster electrons have shorter wavelengths.
• Wider fringes for faster electrons.
• Fringe spacing doesn't depend on electron speed.

2 · Set Up

1. Open the electron-double-slit preset and press Reset. Slit separation = 100 nm, screen distance = 50 cm.
2. Enable the de Broglie wavelength and fringe-spacing readouts.
3. Set the electron's kinetic energy for each trial and record the interference fringe spacing on the screen.

3 · Collect Data

Plot fringe spacing y (y-axis) against wavelength λ (x-axis) for your three trials. Is the line straight through the origin?

4 · Analyze

1. For one trial, compute λ = h/√(2m_eK), then fringe spacing y = λL/d using screen distance L = 50 cm and slit separation d = 100 nm. Compare both to the table.
2. Explain why this is the exact same double-slit fringe formula used for light — once you accept that electrons have a wavelength, their interference pattern obeys identical wave mathematics.

5 · Extend

1. Remarkably, this interference pattern builds up even when electrons are fired through the apparatus one at a time, with long gaps between them. Explain why this rules out the idea that electrons are simply interfering with EACH OTHER, and instead suggests each electron somehow 'interferes with itself.'
2. If you set up a detector to determine which slit each electron actually passed through, the interference pattern disappears — you get two simple bands instead. Why might 'measuring which path' fundamentally destroy the wave interference?

The Physics Behind This Experiment

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