Answer: When the electric field due to one is a maximum, the electric field due to the other is also a maximum, and this relation is maintained as time passes. They alternatively reinforce and cancel each other.
Explanation:
In a wave, the phase, is an arbitrary time reference, used to locate a given point of the wave in time, within a cycle.
Two waves can travel at the same speed, or even have the same wavelength, but this is not enough to be sure that at a given point in time, both waves will be in their maximum, as it only can be determined from the phase of the waves.
So, only when the waves reach at the same point in time at the same amplitude, we can say that they arrive in phase, in a constructive interference.
