When two sonic sine waves of close frequency occur at the same time, we hear them as the average of the two frequencies, with the amplitude pulsing at the rate of the difference between the two frequencies (beats). When they get far enough apart we perceive them as two distinct pitches, depending on their relationship. (We might not, for example perceive two sine waves as separate if one is exactly twice the frequency of the other because of the overlapping harmonic series.)
If two light waves of different frequencies, such as red and green, we perceive them as yellow. All three colors are single-frequency colors. But are red and green somehow averaging into yellow before they reach our eye? Or this is strictly an artifact of our color vision? We can generate a million colors on a computer monitor with just three primary (additive) colors. If red and green both reach our eye at the same time, they excite the sensitivity ranges of the red and green cones that are also stimulated by pure yellow light. So we perceive it as yellow.
Question:
But then again, did the two colors actually combine to average out to yellow by the time they reach our eye? Or is yellow on a computer monitor just an illusion that takes advantage of how our eyes work?
Is there anything in EM waves comparable to beats in sonic waves?
And everything about color perception is vastly complicated by how weirdly our eyes work. The ears are a lot closer to a simple FFT of the incoming time-varying pressure wave. Eyes are not like that at all.
The way human eyes detect light is with particle interactions. Each cell in the retina reacts to the number of photons it receives. It’s purely magnitude, as phase information is lost, so effects like the beat frequency aren’t generally possible.
One exception is with laser light illuminating a surface. It’s possible to see “speckles”, which are small spots of brighter and darker illumination. This is caused by constructive and destructive interference, which is a special case of beat frequencies. But note that this is outside the eye, with the eye merely observing the effect.
Beat frequency is never something that just happens in the detector. The only reason that we never see light beats is that light is so high-frequency, it would be exceedingly difficult to get two sources so close that the human vision system could even detect the beats.
No. If you passed the through a prism you would get the original two colors back. Red photons and green photons. Nothing changes the energy (color) of the photons.
You could also have an animal, or a detector, that had better capability to break up light into frequencies. You could even have one that could fully distinguish between different spectra, though that usually comes at the cost of some of your spatial resolution. Or you could have full spectral and spacial resolution, but at the cost of having to fit what you see to your mental model of the world (we already do something like this with many aspects of our vision). Of course, relying on a mental model like this means that you can be fooled by situations that don’t match the mental model, what we call optical illusions. A creature that had full-spectral eyes would presumably be subject to entirely different categories of optical illusions that we’re not subject to. Though they would also categorize our seeing of a mixture of red and green as yellow as an optical illusion.
On a related note, why don’t two light waves seem to interact?
Say I have a green and red laser. I orient them so that the red laser passes through the beam of the green laser. You would think the changing E fields of the red laser would mess up the M fields of the green laser, and vice-versa. But the two laser beams seem to pass through each other “perfectly,” each unaffected by the other.
Because both E and B fields from different sources add together linearly. In the region where the two beams overlap, you have a complicated mess of E and B fields, but the only way for that mess to evolve is as two beams leaving the mess.
That said, it’s not quite perfect, and there is is actually a little bit of interaction between the two. But until you get up to around a million electron volts (the mass of an electron-positron pair), it’s negligible.
One very arm-wavy way to think about it is that sound waves need a medium. A solid, liquid, or a gas. Which constituent particles have mass, momentum, density, coupling constants, etc.
Conversely, EM doesn’t need an aether. It runs off a much more fundamental set of properties of our universe.
So some wavy aspects work similarly between the two and others do not. Which is which gets beyond my pay grade very quickly. But the idea that there should be both similarities and differences is readily graspable to a layperson.
The presence of a medium for sound waves mostly just means that in extreme cases, they don’t act quite as wave-like as they should, because a medium has things like attenuation and a minimum pressure.