Do coherent waves need the same frequency or just constant phase difference?

[khurram usman]

is it necessary ffor coherent waves to be of same frequency? bcoz everywhere i read its written dat they must have a constant phse difference. but can't they have frequencies dat are simple numerical multiples of each other?
like 256 512

 

[Drakkith]

That depends on what you mean by coherent. Coherence just describes the correlation of the properties of waves. See here: http://en.wikipedia.org/wiki/Coherence_(physics )

 

[abluphoton]

i think yes.. being multiples makes waves beat at the frequency that is HCF of both the frequencies.

 

[sophiecentaur]

There is a constantly changing phase difference between a fundamental and second harmonic. 2πf radians per second, actually. It just happens that, every cycle of the fundamental, the phase is a whole number of cycles.