The double-slit experiment
Predict firstWhen two coherent contributions reach the same place, why do they sometimes add and sometimes cancel?
Connect phase addition, fringe spacing, and single detections.
Prerequisites: Coherent waves keep a stable phase relation; amplitudes add first and the intensity is taken from the sum.Compare the phases first
Predict firstWhen two coherent contributions reach the same place, why do they sometimes add and sometimes cancel?
A path difference produces a phase difference. Contributions in phase add, while opposite phases can cancel. The colors show the signed amplitude in slow motion; the intensity recorded on the screen goes as the square of the total amplitude.
How to observe
- Watch two waves superpose in phase
- Change the slit separation and the wavelength and compare fringe spacing
- See how independent detections gradually build a statistical distribution
Model notes
A far-field model with identical rectangular slits and monochromatic coherent light; the screen intensity includes the single-slit diffraction envelope. The wavefront view is a scalar sketch, not a full electromagnetic field. Detections are sampled from the intensity, and no path through either slit is shown or assumed.