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Is there any non-digital (naturally existing) mechanism to compare two or more waves in such a way:

Input 1           Input 2          ....            Output
-------           --------         ....          ------------
Lower             Higher           ....          Lower/Higher
Wavelength        Wavelength       ....          Wavelength

... some kind of selectively permeable membrane which allows one wave to pass-through?

Pardon my poor physics knowledge. I don't have a clue; would it be related to wave theory, applied physics or applied optics?

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I am curious whether someone understands what you're asking but I have no clue. Looking at the table, I thought that you were looking for an algorithm to manipulate with words and adjectives and put slashes in between them if there are several options. Later, however, we learned that it could be related to membranes (something that didn't occur before) or applied physics or applied optics. So the actual identity of the question is really mysterious and spread all over the Universe, as far as I can see. – Luboš Motl Dec 13 '12 at 21:35
@LubošMotl, as I suspected, its a pretty vague query. Let me try it again: A 'non-digital mechanism'; I am curious if there exists some (mechanical or perhaps biological) construct which, when receives multiple waves, permits one wave to pass through with lowest or highest wavelength? – vulcan raven Dec 13 '12 at 21:52
I suspect you will be disappointed if you seek a natural mechanism that will pass 10 kHz in the presence of 0.1 Hz but block it in the presence of 50 MHz (and similarly discriminate for any arbitrary pair of frequencies). Maybe you seek a naturally occurring tunable notch filter? – RedGrittyBrick Dec 13 '12 at 22:03
In a sense I can see where you are going with "non-digital (naturally existing)" but in another sense trying to draw an equivalence between those two ideas is simply silly: the digital behavior of you computing devices is just as much an expression of natural law as the analog behavior of simpler instruments. – dmckee♦ Dec 14 '12 at 2:24
What are the wavelengths you are interested in? Do you want to separate the wavelengths or actually measure them? – Antillar Maximus Dec 14 '12 at 14:32
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2 Answers

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You may want to look resonance up. There are all manners of physical systems that have a natural oscillation frequency, be this mechanical, optical, or whatever. When excited by a multi-frequency signal, they will amplify their natural frequency more than any other. So in a way you can think of them as blocking all other frequencies.

EDIT

OK, look at this image taken from the wikipedia article... Lets say you have a system with a resonant frequency of 100 Hz. Any real system also has some amount of attenuation due to friction, which is indicated by the $\delta$ parameter. Lets say that our system follows the curve $\delta = 0.2\omega_0$.

enter image description here

So lets now excite this system with a combination of three frequencies: 50, 100 and 200 Hz. The 50Hz excitation will be amplified to about 133% of the input, the 100 Hz excitation to about 240%, and the 200 Hz to only 33% of the input.

As the graph shows, you can actually use any resonant system with strong dampening to filter out the higher frequencies, and adjust the cut frequency with the natural frequency of your system.

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Thank you for pointing me in right direction. When our input waves come in contact with the natural frequency of some fixed standard wave, it will resonate. Incidentally, would this change in frequency of standard wave equals to the wavelength of input wave with highest wavelength or sum of wavelengths of all input waves? – vulcan raven Dec 13 '12 at 22:07
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I'm not sure if it's exactly what you're asking, and arguably it's just a special case of Jaime's answer, but a heterodyne sort of does this.

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