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I am wondering how cones (photoreceptors in the retina) can see color if colors are wavelengths but cones send signals when they absorb photons. Maybe by using the energy level (or only being sensitive to a specific energy level/range)? I suppose if you know E than the brain can provide the function for the wavelength like below? That's an assumption on my part that it's the brain doing the interpretation. Cameras use filters but the human eye seems doesn't seem to use that process other than that the fovea blocks some UV light. Certain cones are certainly sensitive to certain wavelengths, but how are they doing that by absorbing a photon? The energy of it seems to be the only thing that comes to mind.

$$E ~= ~ \frac { hc} { \lambda } $$

Chris
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p1l0t
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    https://biology.stackexchange.com/questions/1446/why-can-cones-detect-color-but-rods-cant is a good place to start. More biology than physics. – Jon Custer May 16 '22 at 22:41
  • Are you asking how wavelength selectivity works in different kinds of cones? – Ruslan May 16 '22 at 22:44
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    The perception of color is in the brain (aka biology) rather than physics. – David White May 17 '22 at 00:13
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    There are questions about color perception on this site. I am not sure if this question is about the physics of how a cone absorbs a photon and how that triggers a neuron. It could also be about how colors arise from triggering neurons in different cones. Either way, it fits this site reasonably well. – mmesser314 May 17 '22 at 00:52
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    Does this answer your question? Seeing colors: photons vs waves – mmesser314 May 17 '22 at 00:55
  • Does this answer your question? What is Gray, from a physics POV? – mmesser314 May 17 '22 at 00:58
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    Thinking of photons and waves as fundamentally different things will get you into all kinds of trouble. Think of it this way: the number of photons is literally the number of units of energy in a wave. So there's really no intrinsic distinction between "absorbing a photon" and "detecting a wave". Another way to think about it is that each photon has a specific wavelength (or range of wavelengths to be a bit more precise). See this question for more details. – DanielSank May 17 '22 at 02:05
  • @mmesser314 I didn't realize I might have to dive into QFT (Quantum Field Theory) but I guess that could make sense to think of it as a field rather than particle vs wave. I guess it could just be easier for a biologist to say "The cone is a transducer and absorbs a photon, done. Moving on.." rather than look into particle physics or QFT. – p1l0t May 17 '22 at 17:45
  • @JonCuster There is definitely a biology side to this. And I did join that community so I can dive down that rabbit hole as well, so thank you for that. The brain and perception is also a key component apparently since rods that everything looks grey pick up 498nm (well centered on 498nm) which is like aqua if it's light enough for cones see. Which I knew the brain is involved but that really drives it home from the selected answer there. That all said, there is definitely a physics end to this that is more what I am interested in overall. – p1l0t May 17 '22 at 18:05

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The interaction between photons and chemistry, eg, via atomic orbitals of single atoms, is a classic tool for understanding quantum mechanics, so there shouldn't be any surprises that the absorbing and emitting of photons is wavelength and photon-energy dependent. For example, this is used in studying spectra from stars, but in lots of other places too.

In the eye, there are opsins (light receptor proteins, e.g., rhodopsin), that absorb light and can consequently change structure, resulting in the eventual activation of neurons. Generally, what is a nice simple spectral line (for, eg, a single atom) will get broadened out in more complicated systems like a protein (ie, complication lead to interactions which lead to broadening). So it's not surprising that the opsins have a broad peak, centered at some wavelength and that peak falls off on either side (although with enough complication, like in a protein, anything can happen, but a single peak isn't a surprise). Given this, it's clear that tweaking the details of the opsins could give absorption peaks centered at different spectral locations, and, voilà,

enter image description here

color vision!

tom10
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  • I'm aware of the rhodopsin/opsin absorbing the photon so are you saying the opsin vary slightly between types of cones and that is why they react to photons with different energies? – p1l0t May 17 '22 at 17:38
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    @p1l0t: "are you saying the opsin vary slightly between types of cones and that is why they react to photons with different energies?" Yes, that is exactly it. (Also, the whole photons vs waves thing is a misdirection here, imho, in that it's not the interesting question. The interesting thing with opsins is what happens once they absorb, but the physics of light isn't special with them. Waves vs photos is often a question with light, and absorption by atoms and molecules tends to generally be easier to conceptualize in terms of photons, but that can be studied in any appropriate system.) – tom10 May 17 '22 at 22:28
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    @p1l0t: btw, there is also a colored oil droplet in cones of some animals too, but I don't know much about it. My assumption has been that is provides a small filtering to tweak the color response curves, but I'm not sure how this interplays with the opsin sensitivity. That different color cones have different opsins is clear. Here's link with a picture of the oil droplets. – tom10 May 17 '22 at 22:40
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    I'm going to accept this as the answer. The references to scientific papers off the opsins wiki link you sent show exactly that and even they don't exactly why the photon makes them move the way they do so I guess that's about as far down the rabbit hole we can go for now. – p1l0t May 18 '22 at 21:01