Interferometric optical testing is based on the phenomena of interference. Two-beam interference is the superposition of two waves, such as the disturbance of the surface of a pond by a small rock encountering a similar pattern from a second rock. When two wave crests reach the same point simultaneously, the wave height is the sum of the two individual waves. Conversely, a wave trough and a wave crest reaching a point simultaneously will cancel each other out. Water, sound, and light waves all exhibit interference, but for the purpose of optical testing, the focus will be the interference of light and its applications.