When Thomas Young did this experiment, he had no laser. He understood it by imagining light to be like waves of water. He thought that light waves moved out from the light source like waves spreading out from a pebble dropped into a pond, and that when the wave fronts hit the double slits, then the original wave gets through at the two slits and there are two different waves from then on. It was easy to figure out how two waves would interact to produce the bright bands and dark bands (often called "fringes") on the screen. He said he had proven the theory that light is waves.