Interference (wave motion)

Interference (wave motion), effect that occurs when two or more waves overlap or intersect. When waves interfere with each other, the amplitude of the resulting wave depends on the frequencies, relative phases (relative positions of the crests and troughs), and amplitudes of the interfering waves (see Wave Motion). For example, constructive interference occurs at a point where two overlapping or intersecting waves of the same frequency are in phase—that is, where the crests and troughs of the two waves coincide. In this case, the two waves reinforce each other and combine to form a wave that has an amplitude equal to the sum of the individual amplitudes of the original waves. Destructive interference occurs when two intersecting waves of the same frequency are completely out of phase—that is, when the crest of one wave coincides with the trough of the other. In this case, the two waves cancel each other out. Intersecting or overlapping waves that have different frequencies or that are not entirely in or out of phase with each other have more complex interference patterns.

Visible light is made up of electromagnetic waves that can interfere with each other. For example, interfering light waves are responsible for the colors occasionally seen in soap bubbles. White light is made up of light waves of different wavelengths; the light waves that reflect off the inner surface of the bubble interfere with light waves of the same wavelength that reflect off the outer surface of the bubble. Some of the wavelengths interfere constructively, and other wavelengths interfere destructively. Since different wavelengths of light correspond to different colors, the light reflecting off the soap bubble appears colored. The phenomenon of interference between visible light waves is exploited in holography and in interferometry (see Interferometer).

Interference can occur with all types of waves, not only with light waves. Radio waves interfere with each other when they bounce off buildings in cities, distorting radio signals. Sound-wave interference must be taken into account when constructing concert halls, so that destructive interference does not result in areas in the hall where the sounds produced on stage cannot be heard. The interference of water waves can be observed by dropping objects in a still pool of water and watching how the overlapping waves interfere constructively at some points and destructively at others.

See also Electromagnetic Radiation; Optics.