Double slit interference patterns with variation in distance between slits. When light passes through two parallel slits, the light that passes through each slit interferes with the light passing through the other slit. Where the light waves meet crest to crest, the light interferes constructively, making a brighter light. Where the light meets out of phase, that is, a crest meets a trough, the light interferes destructively, cancelling out and producing a dark region. When a screen is placed in front of the slits, as here, the pattern is seen as alternating bands of light and dark. The distance between the light and dark patches depends on the distance between the slits.