Double-Slit Interference
Shine coherent light through two narrow slits and watch the exact bright/dark fringe pattern form on a screen — adjust wavelength, slit separation, slit width, and screen distance to see the fringes tighten, widen, and even lose whole orders to the single-slit diffraction envelope.
Preview — sign up free to change the numbers yourself
What you can adjust
- Wavelength
- 380 – 750 nm
- Slit separation (d)
- 0.1 – 2 mm
- Slit width (a)
- 0.05 – 0.5 mm
- Screen distance (L)
- 0.5 – 5 m
Sets the light's color (violet at 380nm through red at 750nm) and, through λ in the interference formula, the fringe spacing — longer (redder) wavelengths spread the fringes further apart.
Distance between the two slits — a larger separation packs the interference fringes closer together on the screen.
Width of each individual slit — kept smaller than the slit separation (the slider stops there). Wider slits diffract less, and their diffraction envelope can suppress ('missing orders') some interference fringes.
Distance from the slits to the screen — a farther screen spreads the same fringe pattern wider (fringe spacing scales linearly with L).
About this simulation
What it shows
Light passes through two narrow slits and builds a pattern of bright and dark fringes on a screen. The brightness along the screen is drawn from the exact equations.
The key idea
Light from the two slits adds up wave by wave. Fringes are spaced about λL/d apart, so longer wavelengths, a farther screen or closer slits spread them out. Each slit's own diffraction shapes the overall brightness and can remove some fringes.
Try this
Raise Slit separation (d) from 0.5 mm to 1 mm: the fringes pack twice as close together, with spacing falling from about 2.2 mm to 1.1 mm.