A Brief History of Time Summary Chapter 4
 Light waves have 'crests' and 'troughs'. The highest point of a wave is the crest, and the lowest part of the wave is a trough. Sometimes more than one of these waves can  with each other - the crests and the troughs line up. This is called 'light interference'. When light waves interfere with each other, this can make many colors. An example of this is the colors in soap bubbles.