define constructive interference.

Constructive interference is a phenomenon in optics where the crest or troughs of two or more waves intersect each other. This leads to increase in the intensity of the output wave.

Explanation:

When two or more waves emitting from different sources travel together, they tend to intersect at some point. So the waves can intersect either at both the crusts position and both the waves in trough position. Thus if the intersecting points of the superposing waves are same, then it will lead to constructive interference leading to formation of bright light.

But if the intersecting points of the superposing waves are different, for example the crust of one wave is intersecting with trough of other wave, then it will lead to destructive interference. The output of the destructive interference is a dark spot in those regions.

Thus, constructive interference increases the amplitude of the superimposing waves as they add up the intensities of crusts of all intersecting points or troughs of all intersecting points.