Answer : Constructive interference
Explanation :
When two waves superimpose on each other, the process is called the interference of wave.
There are two conditions for the interference of the wave
- constructive interference
- destructive interference
We know that the trough is the topmost portion of the wave.
In constructive interference, a trough of one light wave meets the trough of a second light wave hence the amplitude becomes larger while in destructive interference the amplitude of the wave becomes zero.