# What is interference?

When two or more waves overlap and combine to form a new wave this process is called interference.

When two (or more) waves travel simultaneously in the same medium, the total disturbance in the medium is at each point the vectorial sum of the individual disturbances produced by each wave. This phenomenon is called interference. For example, if two waves are in phase, they add so that the wave disturbance at each point in space is increased. This is called constructive interference . If two waves are out of phase by 180°, the wave disturbance in the propagating medium is reduced. This is called destructive interference. If the magnitudes of two out-of-phase waves are the same, the wave disturbance is completely canceled

A special type of interference is produced by two waves of the same frequency and magnitude traveling in opposite directions. The resultant wave pattern is stationary in space and is called a standing wave. Such standing sound waves are formed in hollow pipes such as the flute. It can be shown that, in a given structure, standing waves can exist only at specific frequencies, which are called resonant frequencies. For two (or more) waves to interfere constructively or destructively the phase of one wave with respect to the other must be well defined in time and space. In other words the phase of the disturbances produced by the waves must be correlated in time and space. Sets of waves exhibiting such correlation are referred to as coherent.

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