Double slit — observer on, pattern gone (3D)
About this app
Single quanta arrive as dots on the screen, and thousands of hits grow into the fringe pattern. The detector slider raises which-path information continuously — the fringes fade into two humps without anything changing at the source.
Observation means interaction
In a double-slit experiment with single quanta, an interference pattern forms as long as it cannot be determined which slit each quantum passed through. Once this which-way information is recorded, the fringes disappear. The transition is gradual: the fringe visibility V drops as the measurement gets stronger. In the Fraunhofer approximation the fringe spacing is Δx = λ·L/d; for electrons accelerated through a voltage U the de Broglie wavelength is λ = h/√(2·m·e·U). With full measurement you are left with the wide diffraction pattern of the single slits, not a narrow geometric shadow as with balls.
What the app shows
A schematic 3D scene shows source, double slit, detector and screen; slits and distances are greatly enlarged, while the screen uses the real values. You pick photons or electrons and set wavelength or voltage, slit spacing, slit width and screen distance. Single hits appear as dots, and a theory card plots the curves without detector, with known path, and the geometric shadow of classical balls. The model: distribution = V·interference + (1 − V)·sum of the single-slit diffraction patterns.
Things to try
Start the source and let a few thousand hits accumulate until the fringes are clear. Then raise the measurement strength γ and compare the measured and theoretical visibility. With detectors at both slits the model gives V = 1 − γ. With a detector at only one slit, V = √(1 − γ): even a silent detector reveals the path. Notice that nothing changes at the source while the fringes fade.
Frequently asked questions
- Does the double-slit pattern vanish because someone is looking?
- No. What matters is that a physical interaction leaves information about the path behind, for example in a detector or in the environment. Whether a person reads the result is irrelevant. The more complete the which-way information, the weaker the fringes.
- What is fringe visibility in the double-slit experiment?
- Visibility V measures the contrast of the interference fringes, V = (I_max − I_min)/(I_max + I_min). V = 1 means full fringes, V = 0 means none at all. Values in between describe a partial which-way measurement.
- What is the wavelength of electrons in a double-slit experiment?
- For electrons accelerated through a voltage U, the non-relativistic de Broglie wavelength is λ = h/√(2·m·e·U). At a few hundred volts this is a fraction of a nanometre, much shorter than visible light, which is why electron double slits need very fine slits.
- What can I set in the 3D double-slit observer app?
- You choose photons or electrons, set wavelength or voltage, slit spacing, slit width and screen distance, and adjust the measurement strength γ and the detector position (slit 1, slit 2 or both). Hits, speed and the theoretical and measured visibility are shown in the “Measurement” panel.
Subject: Physics
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