Quantum mechanics
The crisis of classical physics and the quantum hypothesis
Compare how the detection pattern accumulates with one slit versus two slits open; show that individual photons still build an interference pattern — wave–particle duality at the level of single events.
Equipment
- “One slit” / “Two slits” buttons choosing the slit configuration
- “Emit 100 photons” button — a one-photon-at-a-time source
- “Wavelength λ” and “Slit separation d” sliders
- Detection screen recording each hit, with a “Clear screen” button
Procedure
Open one slit and watch the pattern
Choose “One slit” and press “Emit 100 photons” several times. Each detection is a separate dot, yet the accumulated distribution grows into a smooth single-slit band with no fringes — the photons arrive one by one, governed by probability.
Open two slits and wait for the fringes
Press “Clear screen”, select “Two slits” and emit a few hundred more photons. Alternating bright and dark fringes emerge out of random dots: the pattern belongs to each photon's probability amplitude, not to interactions between photons.
Change λ and d, then check the fringe spacing
Raise “Wavelength λ” or lower “Slit separation d” and predict the direction of change of the fringe spacing before looking at the screen. Compare the trend with Young's formula : fringes widen for larger λ or closer slits.