Plane waves arrive from the left and meet a barrier with slits in it. Whatever gets through spreads and overlaps. Where crest meets crest the light is bright; where crest meets trough it is dark. That is the entire mechanism. Everything else on this site is a variation on it.
Both views below are computed live from the same Huygens sum: every point across each open slit radiates a wavelet with its exact path length, and the complex amplitudes are added. Left: the wave field, run at ripple-tank scale so the fringes open to visible angles. Right: the intensity on a screen 1 m downstream at the true scale (micrometre slits, nanometre light), where the fringes are millimetres apart.
Make a guess first. If you push the two slits further apart, do the bright fringes on the screen spread out or crowd together? Decide, then move the slit separation slider and see.
Fringe spacing on the real screen: · predicted λL/d: · single-slit envelope first zero at from centre.
Light of 550 nm through two slits 60 µm apart lands its bright fringes 9.17 mm apart on a screen 1 m away. Drag the underlined numbers, or tab to one and use the arrow keys. The sliders above follow.
Slide the wavelength: fringe spacing grows in direct proportion to λ. This is exactly how Young measured the wavelength of light with sunlight and card, two centuries before lasers.
The formulaMove the slits apart and the fringes crowd closer: spacing goes as 1/d. The pattern is nothing but the geometry of path differences.
Why upside downWiden the slits and the pattern brightens. The catch: a single-slit diffraction envelope closes in and snuffs the fringes out where it goes dark. Narrow and clean, or wide and cramped.
The envelopeExtra slits leave the bright fringes where they were but hone them narrower and brighter. Continue to N in the hundreds and you have built a diffraction grating.
Where this leadsBright fringes sit where the path difference is a whole number of wavelengths. Step through it: four lines of algebra take you from that idea to the spacing you can measure. The numbers under each symbol are the bench's current settings.
Every readout above is measured off the computed pattern, then checked against this formula. They agree because the formula is the far-field limit of the same wave sum.