This simulator recreates Thomas Young's 1801 double-slit experiment: coherent light passes through two narrow slits and produces a pattern of bright and dark fringes on a distant screen, direct evidence that light behaves as a wave.
• A full optical path diagram: an incoming plane wave, a barrier with two slits emitting circular wavelets, and a screen showing the resulting bright/dark fringe pattern rendered in the wavelength's own color. • Three live controls: wavelength (400-700 nm), slit spacing, and screen distance. • A live fringe-spacing readout computed from Δy ≈ λL/d. • Three preset wavelengths spanning blue, green and red light.
Each slit acts as a source of circular wavelets (Huygens' principle). At any point on the screen, light from the two slits has traveled slightly different path lengths. Where that path-length difference is a whole number of wavelengths, the waves arrive in phase and add constructively (a bright fringe); where it is a half-integer number of wavelengths, they arrive out of phase and cancel (a dark fringe). The condition for bright fringes is d sinθ = mλ, where m is the integer fringe order.
Fringe spacing Δy ≈ λL/d grows with longer wavelength or greater screen distance, and shrinks with wider slit spacing. This is why red light produces more widely spaced fringes than blue light under otherwise identical conditions, and why the effect is easiest to see with slits spaced only fractions of a millimeter apart.
It demonstrates that light behaves as a wave: passing coherent light through two narrow slits produces an interference pattern of alternating bright and dark fringes on a screen, which only makes sense if light waves from the two slits are adding and canceling.
Bright fringes occur where the path-length difference between light from the two slits equals a whole number of wavelengths: d sinθ = mλ, for integer order m = 0, ±1, ±2, and so on.
Longer wavelengths (redder light) produce wider fringe spacing; shorter wavelengths (bluer light) produce narrower, more closely packed fringes, following Δy ≈ λL/d.
Increasing slit spacing d decreases the fringe spacing on the screen — the bright and dark bands pack closer together, since Δy is inversely proportional to d.