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Recently i saw a question here which asked "what does the color of light depend on as we percieve it?".Now some members answered that if you see an object from any other medium it appears the same colour as in air.But we are forgetting that light passes through aqueous humor and vitreous humor or some fluids in the eye which changes the wavelength to some "constant" (say).Now this constant will be same as in air or water.basically the fluids act like filter.so how can we explicitly say that color of light depends on frequency?

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The short answer is that the perceived color depends on the impacting photon energy, which is unaffected by changes of refractive index.

A much longer and yet still incomplete answer would be that the exact color "perceived" (i.e. at the consciousness level) is in large part an illusion, depending on an awful lot of factors:

  • biological, e.g.
    • the exact composition and distribution of the colour receptors in the retina's rods, leading both to dischromatopsy (colour blindness or wrong perception of colour) and tetrachromacy,
    • the exact composition of the opsine molecules present in the retina's receptor, which is not the same in all human beings, and also affects relative color perception (i.e., two people may agree that a given frequency is apple green, and yet disagree that a different frequency is deep red).
    • receptor density and efficiency (e.g. phosphodiesterase-6 inhibitors cause cyanopsia, a blueshift in the perceived colours, by keeping blue opsine receptors overstimulable),
    • blood perfusion of the retina (which mostly influences light perception, but colour too),
  • perceptual, e.g.

    • by mixing two colours I may be able to make you perceive a third colour, whose frequency is nowhere near the light actually arriving into your eyes,
    • by rapidly alternating several colours I can do the same (see Newton's Color Wheel)
    • by juxtaposing two different colours I can do even worse.
  • psychological, e.g.

    • some people may see (rather "perceive") colours that are not really there at all (it was a famous quirk of the Nobel Prize Richard P. Feynman); the same effect may be obtained with any one of several psychotropic drugs;
    • some people may perceive different shades of colour depending on mood (the reverse of the common belief that color affects mood)
    • it has been suggested that color perception is also culturally based, so that Homer actually did see the wine-dark sea, and some populations see shades of green that other populations are unable to tell apart.
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We cannot say that color (as perceived) depends only on frequency. It also depends upon color context as these illusions illustrate.

When your brain tries to figure out what color something is, it essentially subtracts the lighting and background colors around it, or as the neuroscientist interviewed by Wired says, tries to “discount the chromatic bias of the daylight axis.” This is why you can identify an apple as red whether you see it at noon or at dusk.

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Some of us aren't forgetting and contributed an answer that made exactly that point. It is an erroneous argument to say that frequency is the determining factor on the basis that the same object viewed in different media appears to be the same colour.

It is meaningless, since the light has to travel through the vitreous humor (with refractive index 1.33) immediately prior to reaching the retina. Thus light of a given frequency will also reach the retina with exactly the same wavelength whatever medium that light has travelled through to get there.

The correct answer could be based on the physiology of the receptors. However, I am no biologist, so I offer one obvious experiment in favour of frequency rather than wavelength determining colour. During a vitrectomy, the vitreous humor is temporarily replaced with other substances, often air or other gases with a completely different refractive index. In none of the few articles I have read, often for the patient's benefit (e.g. here), does it mention any drastic changes in colour perception as one of the temporary side effects of the procedure.

Therefore I deduce that since the frequency of the light is invariant, but its wavelength as it reaches the retina could be changed by up to 30% after a vitrectomy, that it must be frequency that determines colour perception.

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  • $\begingroup$ In fact, it is somewhat useless to speak about properties of light in vitreous humor, since after it the light has to travel some time inside the receptor cell until its intensity goes low enough for light to be considered fully absorbed. $\endgroup$ – Ruslan Dec 10 '15 at 13:25
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The eye has a number of different receptors, that are excited to different levels by light. But even if the light inside the eye were to be transformed, such as for example, we were seeing a negative image, the whole is conditioned to what we see outside, and that colour we call "red" excites the eye and brain into producing a 'red' colour. This can be true even if there is no red in the colour.

Brown is not on the spectrum, it is at best a cross-spectral colour that stimulates the eye in exactly the same way we see any of the rainbow colours (which are single-frequency). But brown is every part of our experience as the other colours.

It really does not matter exactly what colour that you are seeing or that I am seeing. For example, my eye might invert the spectrum, and your eye might make it negatives, but because the reference colour comes from an external colour chart, the reddish-green colour that excited my eye and the deep purple in your eye come from a known blue, and we use 'blue' for that word.

On the other hand a considerable amount of research has been done into colour perception, and people in the tropics are more likely to loose blue (in that the colour is gradually mapped onto adjacent regions like green and grey), then those in higher lattitudes. The number of colours you see depends purely on how you decide to map them.

As an experiment, I mapped 'orange' onto a shade of brown. This is opposite to the Russians, who see what we call 'blue' as two distinct colours (dark blue vs sky blue). Then orange is a kind of brown, one seens the similarities of these in the same way as the two blues.

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The frequency of light doesn't change in different media, the wavelength does. Several factors that can influence your vision. Most objects you see, don't emit light themselves they only reflect light, so the spectrum of the reflected light depends on the light (e.g. when you are diving, everything appears blue because the red part of the sunlight is filtered by water). Media between the object and your eye can alter this spectrum (filter some frequencies) before your eye detects the color (frequency). This information is then processed by your brain (-> optical illusions, see answer from Dirk Bruere).

There is a chapter in the Feynman lectures about color vision.

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