Modern Physics
Electron double-slit
Switch to double-slit mode — fringes scale with λ and slit separation.
Electron double-slit — interactive Modern Physics simulation. Switch to double-slit mode — fringes scale with λ and slit separation. Free browser-based virtual physics lab with live SI measurements and a guided tutorial.
Matter-wave interference
Switch to double-slit mode — fringes scale with λ and slit separation.
Electrons through two slits produce interference like photons, but de Broglie wavelengths for kilovolt electrons are picometers—patterns are fine. The preset exaggerates wavelength on a classroom scale so fringe spacing Δy = λL/d is visible when slit separation or energy changes.
- Δy = λL/d
- Wave–particle duality
Investigation brief
Plan the question before you open the lab
The brief mirrors the prerendered page: driving question, competing predictions, variable roles, governing laws, setup, analysis and extension prompts remain visible and in this order.
Driving question
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?
Predictions to weigh
- 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.
Variable roles
What you set:
- Kinetic energy K (eV)
What you measure:
- De Broglie wavelength λ (pm)
- Fringe spacing y (cm)
How the investigation runs
- Open the electron-double-slit preset and press Reset. Slit separation = 100 nm, screen distance = 50 cm.
- Enable the de Broglie wavelength and fringe-spacing readouts.
- Set the electron's kinetic energy for each trial and record the interference fringe spacing on the screen.
Governing equation
Electron Interference Fringe Spacing — λ = h/p
Just as with light, two coherent wave sources separated by distance d produce interference fringes spaced by y = λL/d on a screen at distance L. For electrons, λ is the de Broglie wavelength h/√(2mK), so faster electrons produce more tightly-spaced fringes.
What the printable worksheet asks students to work out
- 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.
- 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.
Where this shows up beyond the lab
- 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.'
- 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?
- AP Physics 2 — Unit 15: Modern Physics
- IB Physics — E.2 Quantum physics
- General High School Physics — Modern physics intro
- NGSS High School Physics — Wave-particle duality of light
- Welcome to Electron Double Slit
- Select the electron
- Press Play
- Bright and dark fringes
- Open the Properties panel
- You did it!
Open the interactive simulation to build the scene, press Play, and explore with live measurements and a guided tutorial.