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Why do two open slits produce an interference pattern for light?

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A horizontal gradient bar displays the visible light spectrum from violet to red, labeled with corresponding frequency in terahertz, wavelength in nanometers, and photon energy in electron volts. — light When white light is spread apart by a prism or a diffraction grating, the colours of the visible spectrum appear. The colours vary according to their wavelengths. Violet has the highest frequencies and shortest wavelengths, and red has the lowest frequencies and the longest wavelengths.

When light passes through two narrow slits, it produces an interference pattern because of the superposition principle, which describes how overlapping waves combine. As the light waves emerge from the two slits, they spread out and intersect.12

At points where the waves arrive in phase—meaning their crests coincide—they undergo constructive interference, reinforcing each other to create bright bands. Conversely, where the waves arrive out of phase, the crest of one wave meets the trough of another, resulting in destructive interference and the formation of dark bands. This alternating pattern of bright and dark fringes provides clear evidence of the wavelike nature of light.3124

While this phenomenon is classically explained by wave superposition, quantum mechanics reveals that even individual photons contribute to this pattern over time, as the intensity of the pattern at any point represents the probability of a photon arriving there.4