An interference fringe or pattern results from constructive and destructive interference of light waves. By passing sunlight through two narrow slits, the fringe pattern is viewed on a screen placed behind the slits.
Constructive interference occurs when one wave's peak matches the peak from another, creating a situation in which the waves will add to each other. In destructive interference, a peak from one wave pairs with another wave's trough. These waves will neutralize each other. Constructive interference results in a bright line on the screen, while destructive interference results in a dark line. These patterns of bright and dark lines were first discovered by Thomas Young in 1801.