Light does not always travel in a straight line. When it passes through a narrow opening, it bends and spreads outward — and you can see this directly with a laser pointer and two sheets of cardboard. Cut two small parallel slits in one sheet, set up a whiteboard as a screen in a dark room, and shine the laser through. The light spreads well beyond the width of the slits. Where the spreading waves overlap and interfere, they create a pattern of alternating bright and dark bands that is three to four times wider than the single-slit result.