Light energy moves forward in waves. These waves are analogous to water waves, with crests and troughs. If a pebble is tossed into a pool of water, waves radiate outward. If a second pebble is tossed in, where crests from the two sets of waves meet, there is a higher crest. Where the crest of a wave from one source meets the trough from the other, the waves flatten out and disappear.