What happens when light passes through two narrow slits instead of one? You cut a single slit in one piece of cardboard and two parallel slits in another. Then you shine a laser pointer through each card onto a white screen in a dark room. With one slit, the light spreads into a bright band that fades at the edges. With two slits, the spreading light waves overlap and interfere with each other. This creates a pattern of alternating bright and dark bands on the screen. The pattern is three to four times wider than the single-slit result.