Adjust slit separation, wavelength, and screen distance in real time. Watch coherent wavefronts interfere and paint bright and dark fringes — exactly as Thomas Young proved the wave nature of light in 1801.
When wavefronts from the two slits arrive in phase, they reinforce each other — constructive interference. The path difference is an integer multiple of the wavelength.
Position of nth bright fringe:
yn = nλD/d
When wavefronts arrive exactly out of phase (half-wavelength path difference extra), they cancel completely — destructive interference. Zero intensity at these points.
Position of nth dark fringe:
yn = (2n+1)λD/2d
The distance between two consecutive bright (or dark) fringes is constant and called the fringe width β. It increases with wavelength and screen distance, and decreases with slit separation.
All fringes are equally spaced. The central maximum is the brightest.
The overall intensity is also modulated by single-slit diffraction from each slit. Fringes near the edges get weaker and eventually vanish where the single-slit pattern has its minima.
Narrower slits → wider envelope → more visible fringes.
All key concepts, conditions, and formulae you need.
Path diff.: Δ = d·y/D for small angles
where δ = 2πd sinθ/λ is the phase difference between the two slits, and β = 2πa sinθ/λ is the single-slit phase parameter. Maximum intensity is 4I₀ at constructive interference.
Young used a single source S with two holes S₁, S₂ to ensure coherence — the two secondary sources always have the same frequency and a constant phase difference. Modern experiments use a laser beam directly on two slits. The coherence length must exceed the path difference for fringes to form.
When d/a is an integer, some interference maxima coincide with diffraction minima and disappear — these are called missing orders. For example, if d = 2a, the 2nd, 4th, 6th… orders are missing from the pattern.
Thomas Young (1801) performed this experiment to settle the Newton vs Huygens debate on the nature of light. The clear interference pattern could only be explained by the wave theory. Einstein's 1905 photon theory later showed light is also a particle — wave-particle duality. Even single photons fired one at a time build up the same fringe pattern (quantum interference).
Christiaan Huygens proposes that light travels as a wave. Each point on a wavefront acts as a new source of secondary spherical waves — the principle used in this experiment.
Isaac Newton argues light is made of particles ("corpuscles"). His enormous authority suppresses the wave theory for nearly a century.
Thomas Young demonstrates interference of light through two slits, producing alternating bright and dark fringes. This is definitive proof of the wave nature of light and ends the corpuscular theory debate.
James Clerk Maxwell proves light is an electromagnetic wave with his equations, providing the theoretical underpinning for Young's result.
Einstein explains the photoelectric effect using photons — light quanta. Combined with Young's wave experiment, this established wave-particle duality: light is both wave and particle.
Davisson and Germer show electrons also produce interference patterns — matter waves. The double slit experiment becomes the central thought-experiment of quantum mechanics.