A card with one slit is placed in front of a card with two slits which is followed by a screen. When a beam is passed from a light source falls on the card with one slit, the light from the single slit produces coherent light at the second card. When the light passes the card with two slits, alternate dark and light bands show up. An interference pattern appears on screen. The dark bands show destructive interference. The bright bands show constructive interference.