Interference.

Consider a situation where we superpose two waves. Naively, we would expect the intensity (energy density or flux) of the resultant to be the sum of the individual intensities. For example, a room becomes twice as bright if we switch on two lamps instead of one. This actually does not always hold. A wave, unlike the intensity, can have a negative value. If we add two waves whose values have opposite signs at the same point, the total intensity is less than the intensities of the individual waves. This is an example of a phenomena referred to as interference.