# Which waves can have a constructive interference?

## Homework Statement

Which waves can have a constructive interference?

∆x=kλ

## The Attempt at a Solution

I think that waves with the same frequency and phase can have a constructive interference. What if they only have the same frequency? Can they create constructive interference?

## Answers and Replies

Merlin3189
Homework Helper
Gold Member
I think that waves with the same frequency and phase can have a constructive interference. What if they only have the same frequency? Can they create constructive interference?
They can. They will interfere constructively in different places depending on their phase.

If two sources have the same frequency and are in phase, where do they interfere constructively?
Why do they interfere constructively there and not in some other places?

They can. They will interfere constructively in different places depending on their phase.

If two sources have the same frequency and are in phase, where do they interfere constructively?
Why do they interfere constructively there and not in some other places?
They interfere construictively at the points in which they meet I think
In Wikipedia it says "Constructive interference occurs when the phase difference between the waves is a multiple of 2π"

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Merlin3189
Homework Helper
Gold Member
They interfere constructively at the points in which they meet I think
Well they interfere at points where they meet, but not always constructively. Sometimes they interfere destructively and mainly it's something in between - partially constructive or partially destructive..
And in general they don't just meet at a few points, they meet everywhere both waves can get to.

In Wikipedia it says "Constructive interference occurs when the phase difference between the waves is a multiple of 2π"
Yes. Because a phase difference of 2π for a sine wave is exactly the same as the original. And another 2π is the same again, etc.

Now, how do these phase differences arise, if both sources are in phase?
And if you have two sources which are not in phase, but differ by say 1π, how will you get a point where they interfere constructively?