Diffraction explains the formation of interference fringes by allowing light waves to spread out and overlap when they pass through narrow openings or slits. When coherent light (light with a constant phase difference, like that from a laser or a monochromatic source) passes through two or more slits or edges, the diffracted waves from each opening overlap in space. Where these overlapping waves meet, they interfere with each other.
At some points, the waves add up (constructive interference), forming bright fringes. At other points, the waves cancel out (destructive interference), creating dark fringes. This alternating pattern of bright and dark bands is known as an interference fringe pattern.
Diffraction makes this overlap possible by bending the light into regions where it can meet with light from another slit. Without diffraction, the light would travel in straight lines and not overlap, so no interference pattern would form.
Thus, diffraction is the mechanism that enables light waves to spread and interact, while interference is the result of their superposition—together, they explain the observed pattern of fringes in experiments like Young’s double-slit experiment.