Path difference determines bright and dark fringes
When light from a coherent source passes through two narrow slits, it diffracts at each slit and the diffracted waves interfere on a distant screen. At points where the path length from one slit exceeds the other by an integer multiple of the wavelength (delta L = m lambda), the waves arrive in phase and interfere constructively, producing bright fringes. At points where the path difference is a half-integer multiple (delta L = (m + 1/2) lambda), the waves arrive out of phase and cancel, producing dark fringes. The spacing of fringes depends on wavelength (smaller lambda means finer spacing) and the geometry (smaller slit separation or larger distance to screen means wider-spaced fringes). This pattern is an unambiguous signature of wave behavior: a particle passing through either slit would produce two distinct spots, not fringes.
The measurement paradox and quantum mechanics
If you try to observe which slit the light passes through by placing a detector at one slit, the interference pattern vanishes and you see two separate bands instead of fringes. This is not because the detector disturbs the light (though it does) but because measuring 'which slit' forces the light into particle-like behavior. Quantum mechanically, light (and electrons) exist in a superposition of both paths until measured. A which-way measurement collapses this superposition, eliminating interference. This is one of the most counterintuitive results in physics: the act of measurement fundamentally changes the outcome, showing that reality at the quantum scale does not have properties independent of observation.