Before I get a bunch of answers talking about the speed of light varying when traveling through different mediums and capacitors and resistors being used to filter EM waves on a radio, I understand all of that. What I don’t understand is how one wave can be easily separated from another wave. For example, let’s say I have a 5Hz and 10Hz wave generator in water. I’m watching the waves propagate from the generator. As an observer of this, how can I know what signal is coming from the 5Hz generator and what signal is coming from the 10Hz generator? To me, it looks like the amplitude of a 10Hz wave is just doubling every other time.
In: Physics
The wave isn’t defined by it’s value at a point (what you see if you watch the water go up and down at one spot), it’s defined by the gradients around the point. EM waves can (and do) happily interfere with each other so that the value of the electric and magnetic fields at any particular point can vary all over the place but the fundamental waves are all still there, they’re just superimposed. Something like a prism lets them spread spatially out so they’re not interfering with each other and you can actually see each one. From a quantum mechanical standpoint they’re all still interfering with each other, but that’s beyond the scope of the question.
In the water example, you have no way to know it’s not one generator that’s producing 10Hz with alternating amplitude, but if you *assume* that you’ve got some number of pure sine wave generators then you just do a Fourier transform on the water height signal and it’ll spit out spikes at 5Hz and 10Hz. For EM, we don’t have to assume it’s a pure frequency, we know it is, because that’s how photons work. Any individual photon can only have one frequency.
Water is a molecular medium, so your water waves, like air waves, are transmitted as a sum. You can’t separate that sort of signal, there is no real equivalent to a Fourier Transform.
EM waves are totally different. As a quantum phenomenon, individual photons have their own properties. Red photons and blue photons are distinguishable by their different energy levels. There are no “white” photons, white light is a mix of colors. The energy level influences refraction behavior, this is the principle upon which a prism works.
I can also take a crack at answering this myself. My thoughts are that the wave constructed on water is not comparable to EM waves in this respect. When we are looking at EM waves, there are different photons, each oscillating at a different rate. So it’s easy to me to imagine a filter that looks at the photons on a “case by case” basis and filters them based on their individual oscillation frequency. This is not possible with the water because it is not propagated by photons and thus I believe that the water wave cannot be deconstructed into it’s constituent waves. I just don’t know if this explanation is correct, or just some mumbo jumbo that I came up with to understand this in my brain.
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