Double-Slit Interference
Adjust slits, see pattern. Builds on Traveling EM Waves.
You shine a laser beam through two very narrow, closely spaced slits onto a distant screen. Instead of seeing two bright lines (one from each slit), you see something unexpected on the screen.
What pattern appears on the screen?
You see many evenly spaced bright and dark fringes! This is the hallmark of wave interference. Light waves from each slit spread out and overlap. Where the path difference is a whole number of wavelengths (nλ), the waves add up → bright. Where it’s half a wavelength off ((n+½)λ), they cancel → dark. This pattern proved that light is a wave.
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Watch wavefronts from both slits spread out and overlap. See how constructive interference (bright) and destructive interference (dark) create the fringe pattern on the screen.
d sinθ = mλ
Bright fringes occur when the path difference from the two slits equals a whole number of wavelengths. More separation → closer fringes. Longer wavelength → wider fringes.
Thomas Young’s double-slit experiment was the first conclusive evidence that light is a wave, settling a debate that raged since Newton (particles) vs Huygens (waves).
Even single electrons fired one at a time through two slits build up an interference pattern over time. This is the deepest mystery of quantum mechanics.
Phone screens and camera lenses use thin film interference (same principle) to reduce reflections. The film thickness causes destructive interference for reflected light.
Astronomers use the double-slit principle with widely separated telescopes to measure star diameters with incredible angular resolution.
“Two slits, one laser, and a screen — that’s all it takes to prove light is a wave. And when you do it with single particles, you discover the strangest truth in all of physics.”
Double-Slit Lab
Full control over Young’s experiment. Adjust wavelength, slit separation, and screen distance. Watch the interference pattern change in real time. Try different colors to see how wavelength affects fringe spacing.
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Start with red light (λ = 650 nm) and moderate slit separation. Note the fringe spacing. Now switch to blue light (λ = 450 nm) — the fringes get closer together. Double the slit separation d — the fringes get closer too. Now increase the screen distance L — the fringes spread out. Try white light and see the rainbow-colored fringes!
The fringe spacing Δy = λL/d tells the whole story: longer wavelength → wider fringes, larger screen distance → wider fringes, larger slit separation → narrower fringes. Interference is geometry meets wave physics.