When a compact disc is illuminated by a source of white light, coloured lines are observed. This is due to

Diffraction is the process of bending and spreading of waves when they meet an obstruction. Also diffraction grating is a reflecting or transparent substance whose surface contains fine parallel grooves or rulings that are equally spaced. When light is incident on grating coloured pattern are formed due to diffraction. When a compact disc is illuminated, then coloured lines are observed because as one surface of the CD has many small pits in the plastic, arranged with concentric rings, that surface has a thin layer of metal applied to make the pits more visible. These pits function as diffraction gratings.