Interference active
What to notice
One photon makes one dot. A wave-like pattern appears only after many repeated detections.
Wave–particle laboratory
Double-SlitInterference
Send photons one at a time. Each arrives as one dot, but many dots gradually reveal a wave-like interference pattern.
1Emit2Observe3Detect
Build the pattern
Change one control at a time, then add photons and compare the screen.
Wavelength λColour and fringe spacing
VioletRed
Slit separation dDistance between the two openings
CloseFar apart
Which-path detectorLearn which slit each photon used
Detected photons0
Fringe spacing1.00×
Paths unknownThe two alternatives remain indistinguishable, so their amplitudes interfere.
Particle-like arrival
The screen never receives half a photon. Every detection is a localized dot.
Wave-like probability
The probability of landing at each position contains bright and dark fringes.
fringe spacing ∝ λ / d
Information matters
When the paths become distinguishable, the interference term disappears—even without watching the data.
Try it
Discuss the result →Can you make the fringes spread farther apart?
Test wavelength and slit separation separately. Which change has the effect you predicted?