Constructive & Destructive Interference
This first diagram shows what happens when two waves, that are in phase,
combine (interfere). In the middle portion where they overlap the intensity is
increased - we call this constructive interference.
In the second diagram the two waves are out of phase. In the middle portion
where they overlap the intensity is zero - we call this destructive
interference.
The third diagram shows an intermediate case (between constructive and
destructive), the two waves being about 90° (i.e. π/2 or λ/4) out
of phase. In the middle portion where they overlap the intensity is slightly
increased.
Obviously other intermediate phase differences will yield other results such as
various decreases and increases in intensity.
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