Lab 15 — Quantum Mechanics

The Double Slit — Wave & Particle

A quantum seen as both a “line” (a wave) and a “point” (a particle) at once. Fire quanta (electrons or photons) one at a time from the source on the left, through two slits, onto the screen on the right. Every single one is detected as exactly one point — yet let thousands pile up and the dots trace out bright-and-dark interference fringes. A particle, interfering like a wave with itself. The shimmer in the background is the wave (the wavefunction); the dots accumulating on the screen are the particles. Two faces of the same quantum.

I(\theta) = I_0\,\cos^2\!\left(\frac{\pi d \sin\theta}{\lambda}\right)\operatorname{sinc}^2\!\left(\frac{\pi a \sin\theta}{\lambda}\right) \qquad \Delta y = \frac{\lambda L}{d}

Grow N and watch the dots become fringes. Hit “Compare” to stack don’t-look vs see-the-path — the only difference is knowing which slit

Slit separation dd = 72

Wavelength λλ = 12

DetectedN = 0

The deepest strangeness: the fringes vanish the instant you observe which slit the quantum took — not because you jostle the particle, but because pinning down the path destroys the superposition (coherence) of the two waves. And the fringes appear even when quanta go through one at a time: a single quantum passes through both slits “at once” and interferes with itself. In 1989 Tonomura and colleagues sent electrons one by one and filmed the dots slowly growing into fringes. This page’s “line = wave, point = particle” is the same idea as the hydrogen electron cloud (sampling |ψ|² one point at a time).