Double-Slit Interference, explained.
Double-slit interference arises when coherent waves from two openings superpose with position-dependent phase differences.
Why it happens
At one-metre screen distance the small-angle intensity is proportional to cos²(πdy/λL). Fringe spacing is λL/d; a one-slit baseline removes this two-source modulation.
Double-slit interference arises when coherent waves from two openings superpose with position-dependent phase differences.
Read the result
Read bright and dark bands as normalised intensity. The one-slit baseline removes modulation without modelling a finite-width diffraction envelope.
A worked example
Pack fringes by separating the sources
Set wavelength to 550 nm and slit separation to 0.25 mm.
Double separation to 0.5 mm while holding the one-metre screen distance fixed.
Compare fringe spacing falling from 2.2 mm to 1.1 mm rather than assuming a brighter source changed the spacing.
OPTIONAL DEEPER DETAILGo deeper: inside the model
Inside this model
At one-metre screen distance the small-angle intensity is proportional to cos²(πdy/λL). Fringe spacing is λL/d; a one-slit baseline removes this two-source modulation.
Where this idea is useful
A practical use
Use fringe spacing to distinguish the effects of wavelength and aperture separation.
A common misconception
“A wider separation must spread the interference bands farther apart.”
The small-angle fringe spacing is wavelength times distance divided by slit separation, so greater separation packs bands closer.
What this explanation leaves out
- Infinitesimally narrow coherent slits and a far-field approximation omit the single-slit diffraction envelope. This is not a particle-detection or which-path experiment.
Where are individual photon impacts in this display?
This is an ideal continuous wave-intensity calculation, with no particle detection, counting noise or which-path apparatus.
Which parameter changes phase difference across the screen rather than merely overall brightness?
Associated thinkers
Associations marked provisional are awaiting source review.
Further reading
Explore the original research or the teaching reference behind this experiment.