When laser light shines on a screen after passing through two closely spaced slits, it becomes

diffracted into semicircular waves. constructive interference occurs where the waves are crest to crest or trough to trough, destructive interference occurs where they are crest to trough. The light that falls on the screen produces bands of light and dark fringes on the screen as a result of these constructive and destructive interferences. This is called the young's slit experiment.