Dr. Thomas Young (1773-1829), an English physicist, confirmed through tests that light waves cause interference. Dr. Young used a screen to confirm the interference pattern consisting of bright fringes and dark fringes that resulted when light from one source was split into two light waves with the same wavelength, which were then made to overlap. Interfering in phase causes brightness, while interfering out of phase causes darkness.

Two waves interfering with one another are called "coherent." Waves that do not interfere in this way are "incoherent." Dr. Young used light from the same source, so he was able to reproduce a coherent state. With a soap bubble, there is interference between countless light waves, but the person creating the soap bubble may see the bubbles in an incoherent state, with no color at all. Incidentally, lasers emit coherent light.