Young's Double-Slit
Adjust λ, d, L — see fringes. Builds on Huygens' Principle.
You’re performing Young’s double-slit experiment with a laser. You see a beautiful pattern of bright and dark fringes on the screen. Now you cover one of the two slits.
What happens to the pattern?
The fringes vanish! Interference requires waves from BOTH slits to overlap and create constructive/destructive patterns. Cover one slit and there’s no second wave to interfere with. You’re left with a broad, smooth single-slit diffraction pattern — a wide central bright region without the sharp alternating fringes. This proves the fringes come from interference between the two slits, not from either slit alone.
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Start with both slits open and observe the fringe pattern. Now cover one slit — watch the sharp fringes dissolve into a smooth diffraction envelope. Uncover it and the fringes snap back. Toggle rapidly to see the dramatic difference.
d sinθ = mλ (bright fringes)
The fringe spacing depends on λ/d. Wider slit separation (larger d) gives closer fringes. This is opposite to single-slit diffraction, where wider slits give narrower patterns. Don’t confuse slit separation d with slit width a!
Thomas Young used sunlight through a pinhole to create a coherent source, then split it through two slits. The resulting fringes were the first definitive proof that light is a wave, overturning Newton’s particle theory.
Electrons fired one at a time through a double slit build up an interference pattern over many detections. This demonstrated wave-particle duality and remains one of the most profound experiments in physics.
Modern interferometers (like LIGO) use the same path-difference principle as the double slit. By detecting sub-wavelength changes in path length, LIGO observed gravitational waves in 2015.
Arrays of radio telescopes (like the VLA) act like enormous double slits. By combining signals from telescopes separated by kilometers, they achieve the angular resolution of a single dish that large.
“Two slits, one of the most important experiments in history. It proved light is a wave, foreshadowed quantum mechanics, and showed that interference — the combination of waves — creates patterns no single source can.”
Young’s Double-Slit Lab
Perform the double-slit experiment with adjustable slit separation, slit width, wavelength, and screen distance. Cover one slit to see the difference. Explore the interplay between interference and diffraction.
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Start with narrow slits and red light. Note the fringe pattern. Increase the slit separation d — fringes get closer together. Decrease the wavelength (switch to blue) — fringes also get closer. Now widen the individual slits — the broad diffraction envelope narrows, eventually hiding the outer fringes. Cover one slit and watch the sharp fringes vanish, replaced by a smooth single-slit pattern. Uncover and they snap back!
The double-slit pattern is interference fringes modulated by a single-slit diffraction envelope. Slit separation d controls fringe spacing (λL/d). Slit width a controls the envelope (λL/a). Both are needed to fully describe the pattern.