In Young’s 1801 double-slit experiment, what pattern appeared when light passed through two narrow slits?

In Young’s 1801 double-slit experiment, light produced interference fringes after passing through two narrow slits.

Each slit acted as a source of waves. Where the waves arrived in phase, they reinforced one another and formed bright bands; where they arrived out of phase, they canceled and formed dark bands.

The result supported the wave theory of light, although the experiment is also famous because single particles can build up an interference pattern one at a time. That later observation became important in quantum mechanics.

The pattern is not simply two bright images corresponding to the two slits. Its alternating bright and dark structure is the defining signature of interference, and the spacing depends on wavelength, slit separation, and screen distance.