Let us consider a phase front corresponding to the ray  and passing through the point  . This phase front also meets the ray  at point  . In other words, the phase of the ray at  (just before the reflection) is same as that of the ray at point  . That is to say that the phase change corresponding to the distance  added with the phase  of the reflection coefficient at points  and  should be a multiple of  . This is what is called the condition for the constructive interference.
From simple geometric considerations we have
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