Shining a laser through two narrow slits produces a result that a single slit cannot. You cut one slit in a piece of cardboard and two parallel slits in another, then shine a laser pointer through each 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 — combining in some spots and canceling in others. The result is a pattern of alternating bright and dark bands that is three to four times wider than the single-slit result.