Interference, in physics, the effect caused by the coming together of waves, including water, sound, and light waves. Interference can be seen when two different series of waves come together on the surface of a body of water. When the colliding waves are in phase (the crest of one coinciding with the crest of the other), the elevation of the water surface is the sum of the heights that the separate waves would have had. When the colliding waves are out of phase (the crests not coinciding or the crest of one coinciding with the trough of the other), there is much less disturbance of the water surface, and the elevation or depression amounts to the difference between the height of one wave's crest and the depth of the other wave's trough.

Similar effects result from the coming together of sound waves and of light waves. When two sound waves of slightly different frequencies are superimposed the loudness of the resulting sound alternately increases and diminishes, producing beats. Beats are heard, for example, when two instruments playing the same note are not in tune with each other.

When two beams of identical monochromatic light (light of one wavelength and hence of one color) are brought together, interference results. Where the waves are in phase the light is intensified; where the waves are out of phase the light is less intense or extinguished. These bands of light and darkness are called fringes.

When two beams of white light (a composite of all colors) come together, each color produces its own interference fringes. The combined effect of all the colors is a brilliantly colored pattern of interference fringes. The iridescent colors of a soap bubble or of a film of oil on water are caused by the joining together of two beams of white light. These are slightly out of phase, one beam being reflected from the outer surface of the film, the other from the inner surface.