Double-Slit Interference

Each slit acts as a source of spreading waves. At any point on the screen, light from the two slits has traveled slightly different distances to get there, so the two waves arrive out of step by some amount. If that extra distance (the path difference) is a whole number of wavelengths, the waves arrive crest to crest and add up: a bright fringe. If it is a half wavelength off, crest meets trough, and the waves cancel: a dark fringe. Every point on the screen falls somewhere between these two extremes, which is why the pattern is a smooth series of bright and dark bands rather than just two spots.

If you watched the pattern build photon by photon, each single photon still lands somewhere and the bright fringes are just where photons land most often, dark fringes where they almost never land. The spacing between adjacent bright fringes depends on wavelength, slit separation, and screen distance: fringe spacing equals wavelength times screen distance divided by slit separation. This is why longer wavelengths or closer slits spread the fringes farther apart.

This topic sits right after single-slit diffraction and the two interact: each slit alone would spread light into a broad diffraction pattern, and the double-slit fringes actually appear inside that broader envelope, shaped by it but not caused by it. Sending the setup underwater shrinks the wavelength of light inside the water, which shrinks the fringe spacing without changing slit geometry, linking this topic directly to how wave speed and wavelength change in different media.