Few questions about waves

[inquisitive1]

i have got few questions to ask about waves
first do waves only interfere if they have 180 degrees phase shift or are inphase? like for example in the image below would these two waves not interfere? if no then why ?

someone on this forum said that orthogonal signals dont interfere as they are 90 degrees out of phase. they have to be 180 degrees out of phase or inphase to interfere

second question is that i was told by someone that interference is a temporary effect. it doesnt last. i.e waves interfere as long as they are superimposed and then they go on traveling to their destination unaffected after the contact between the two waves is over
i wonder when they superimpose each other and gave rise to a new composite wave then why didnt it affect their orginal waveform?

 

[tzushky]

waves

For what I knew, interference of two waves is their sum. The classical examples I had in minde were the two pulses, either same amlitude or one positive one negative moving in opposite directions. You get the result of their interference in time at each instant by adding them. So you get constructive interference when they are both positive and add up, and destructive when nothing is left because they perfectly cancel out.

But also the idea is that waves do not change themselves, they do not disappear. If after a while one of them takes a different path, it keeps all caracteristics as before meeting the other. Interference I believe affects the medium at their encounter spot.

But that's slightly outside the subject...

I'd say that waves interfere no matter what their pahse difference is. So your two waves interfere (add them and you get the interference result).

The thing is that the easiest way to imagine interference is seing trains of waves go one over the other and their addition result. I remember that the best exmaples to understand were visual: water waves or light waves... But water is still best.

Destructive and constructive interference are just special cases of interference, extreme results. That's why they get named all the time. When ripples coming from two sources , on water, collide, they have points of high and low ... theose are the extremes of constructive/destructive interference, if not in all the other points, they are still result of interference, the addition of the two waves, each of them 2 dimensional, for which is you had the equations in space and added the values in that point (x,y) in space you would get the resulting "amplitude".

The phase issue is: when two sinusoids are 180 apart, they are in opposition, so their sum is 0. But if it is the same sinusoid , same frequency. Same goes for 0 dephasing=> constructive. If they are 90 degrees apart... try a simulation.

Why after superimposing they continue safely and unchanged? because each wave has a direction, a frequency, dictated by its source. Sound waves alter the air's molecules' movement, but they hold on to their direction. If someone were to call you from behind a crowd talking loudly, it would mean that you would never hear your name. But you do. ( The fact that your ear has a special filter and a habit of hearing your name called is the second part, it's the reception...)

That means that the wave of sound carrying your name , even after colliding with the others, kept it's direction and strength (as much as air does not trouble it, because waves suffer from attenuation, etc because of their transmission mediums...)

I honestly do not know if mathematics proved it, it's beena few years now... But I think that if you write the equations of two waves thoroughly with all parameters (time and space normally) and add them, you can still se that the result is made of two, so even after time passes, you can see that the sum is made of two separate components, so if space directions differ after a while, they will continue unharmed

I'll try and get back with more solid stuff if I can

 

[A.Anand Srinivasan]

Re: waves

the signals in the figure do interfere... the interference you are talking about when the signals are inphase and out of phase are constructive and destructive interference respectively where in one case the signals add up and the other case one gets subtracted from the other......
orthogonal frequency do interfere... what you heard about would have been meant in the context of radio because the two orthogonal signals can be separated perfectly at the receiver by correlator because one is a sine wave and other is a cosine wave and hence effectively they can be said not to have interference....
talking about the permanence of interference... it is not permanent because the wave though contains oscillations their direction of propagation is perpendicular to it and at points where they meet the amplitude gets added and after that the signals return back to normal...

 

[tzushky]

waves

I have a more visual example of signals of different frequencies interfering, so you can see that it is (obviously as it seems now) possible

**broken link removed**

Also AA Srinivasan is right, 90 degree shifts are inportand in radio signals because receivers can differentiate the two quadrature signals by a phase shift and use them internally for reception... The sin and cos of same frequency are signals that have a 90 degree shift as relationship (without initial phase ...just cos(wt) and sin(wt))

 

[inquisitive1]

Re: waves

thanks alot guys
tzushky it was a great reply
and so was yours anand
btw i asked my question in context of radio waves which i forgot to mention
anyways it was very helpful
thanks guys

 

[A.Anand Srinivasan]

Re: waves

what is your question about radio waves can you please post it.....