Waves

[Lizwi]

If constructive interference results in 0 amplitude what about the waves that interfere and results in lower amplitude ten individual waves. What do we call that interference?

[DrP]

 

From google of 'destructive interference':

 

destructive interference:- Physics - The interference produced when two or more waves are not in phase and create a wave that is weaker than either of them.

 

[Strange]

53 minutes ago, Lizwi said:

If constructive interference results in 0 amplitude

That would be destructive interference.

Constructive interference is when the phases of the two waves add together to give an amplitude that is greater than either one (up to a maximum of the sum of both).

In destructive inferences, the waves have (to some extent) opposite phases and so the amplitudes subtract giving a lower (possibly zero) amplitude.

[Lizwi]

Thanks very much, the high school physics textbook I'm using says destructive interference produce is 0 new wave or pulse. That's why I ask because this sounds incorrect.

[DrP]

23 minutes ago, Lizwi said:

Thanks very much, the high school physics textbook I'm using says destructive interference produce is 0 new wave or pulse. That's why I ask because this sounds incorrect.

If you picture 2 sin waves perfectly out of sinc so that the peaks of one overlap the troughs of another the result of the destructive interference will be 0 as the resultant peaks and troughs cancel each other out.  If they are perfectly in sinc then you will get the amplification of them when the peaks overlap.  If they are randomly out of sinc then you will get a new curve/wave which is a culmination of the 2 waves built by the constructive and destructive interference of the 2 waves as they interact.  

Just as an example - it is how noise cancellation works.  You have a device that receives sound waves in and inverts them and re emits them to interfere with the initial wave... the 2 waves cancel each other out and you get silence.   I hope that helps.  

[Strange]

Does this diagram help:

When the waves have the same phase (in phase) they add together. When they have the opposite phase then the effectively subtract (because one is positive while the other is negative).

Form here: http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/interf.html

47 minutes ago, Lizwi said:

Thanks very much, the high school physics textbook I'm using says destructive interference produce is 0 new wave or pulse. That's why I ask because this sounds incorrect.

Yes, that is correct: destructive interference results in a wave of zero amplitude.

Constructive interference would result in a wave with twice the amplitude.

[DrP]

1 minute ago, Strange said:

Yes, that is correct: destructive interference results in a wave of zero amplitude.

Constructive interference would result in a wave with twice the amplitude.

...assuming they are perfectly in sinc or 180^o out of sinc like in your diagram.  There are many possible combinations of the 2 waves being out of sinc by 30^o , 37, 45, 72.8^o etc.. where you will get both constructive and destructive interference along the wave as you add or subtract the amplitudes of the waves at each point along it which will give a more complex wave pattern than just doubling or total negation of the wave's amplitude.  

[Strange]

16 minutes ago, DrP said:

...assuming they are perfectly in sinc or 180^o out of sinc like in your diagram.  There are many possible combinations of the 2 waves being out of sinc by 30^o , 37, 45, 72.8^o etc.. where you will get both constructive and destructive interference along the wave as you add or subtract the amplitudes of the waves at each point along it which will give a more complex wave pattern than just doubling or total negation of the wave's amplitude.  

Absolutely. I seem to remember a nice animation in another thread, which showed this. I don't know if I will be able to find it again though!

Here it is. A blue wave and a green wave have a changing phase relationship. The result is the red wave. When they are in phase, the red wave is twice as large, when they are out of phase, the red wave is zero:

Form here: https://en.wikipedia.org/wiki/Wave_interference

[DrP]

Yep - That's what I was trying to describe exactly.