Wavelength corresponds to colour, but how?

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Between the input (wavelength) and output (colour), what occurs? How do we receive 650nm, for example, and interpret it as red? What even is red? What is colour, really?

In: Physics

Out eyes have four types of light-sensing cell, the fourth of which is not important for this question. The other three detect a range of frequencies, which normally correspond to a set range of frequencies which we lump together as ‘red, green, and blue’. A red object’s light is mainly absorbed by the ‘red’ cells, and so on.

Our brain interprets the signals from these cells to produce color. Therefore, color is an experience like taste, while frequency range is an objective and measurable fact.

Inside your eyes are special cells called cones that detected light and can distinguish between colors. There are three types of cones and each is sensitive to a different wavelength of color. You can see a graph of the sensitivities [here](https://en.wikipedia.org/wiki/Cone_cell#/media/File:Cone-fundamentals-with-srgb-spectrum.svg).

Your brain determines what color something is based on what specific types of cone cells are being activated.

Color is surprisingly complicated!

There are a few related, but separate, ideas you could call ‘color’:

– Wavelengths of light: The distribution of different frequencies of light entering your eye. This is a complicated distribution that has more data than your color vision can perceive. For example, you might have one level of brightness at 682 nm, another at 573 nm, another at 562, another at 478, another at 426, and so on. This is called the “spectral power distribution”.

– Color *vision*: The amount that cone cells in your eye are stimulated. If you have normal color vision, you have three types of cone cell in your eye. One responds strongly to short-wavelength blue light, one to medium-wavelength green, and one to medium-long wavelength greenish-yellow, with their sensitivity falling off quickly to either side. “Red”, in this model, means “stimulates the greenish-yellow-sensitive cones a moderate amount and mostly doesn’t stimulate the green or blue-sensitive cones”. This layer takes the distribution of frequencies entering your eye and boils them down into three numbers describing how much each cone type is stimulated, called the “tristimulus values”. Interestingly, some combinations of stimulus can’t be achieved by any mix of actual light frequencies, meaning that there is a “greener green” than any green you’ve ever seen that you’re physically capable of perceiving but could never actually see. (Users of psychedelic drugs often report seeing such ‘impossible colors’).

– Color *perception*: Your perception, as a thinking thing, of what objects are ‘the same color’ as what other objects. This takes the previous layer and runs it through some really complicated filtering that accounts for things like ambient light. Optical illusions take advantage of that filtering – see, for example, the [checker shadow illusion](https://en.wikipedia.org/wiki/Checker_shadow_illusion). Under normal circumstances, though, those corrections help you recognize that a white wall illuminated by yellow light is “really” white (even though your eyes are receiving yellow light).

– Color *of a material*: The properties of a material cause it to emit or reflect certain wavelengths more or less than others. If an object reflects a lot of short-wavelength (“blue”) light but not other colors, we say that the object “is blue”. The color of an object is the color you perceive when the object is illuminated by white light without any surrounding context.

Color is just how your eyes react to a particular wavelength. For most people, the reaction to a particular wavelength is the same, so we all agree that wavelength correspond to a certain color (ex. red), just like if scientists measure a wavelength of 1cm, we all agree that this is a microwave. People who are colorblind has a different response in their eye so they can’t agree with the rest of us on that color.

Another analogy would be that for most people, getting pinched (input) is a “painful” sensation (response), getting tickled (input) is a “funny” sensation (response). For certain people who can’t feel pain, they have a hard time relating.

Beautiful GIF:

[https://en.wikipedia.org/wiki/Wavelength#/media/File:Light_dispersion_conceptual_waves.gif](https://en.wikipedia.org/wiki/Wavelength#/media/File:Light_dispersion_conceptual_waves.gif)

And helpful chart:

ELI5: Your eye has three alarms. One goes off for those tight short-wavelength frequencies, one for the middle, and one for the middle-high.

There is a peculiar property of chemicals that a few of them react only with specific wavelengths of light.
Think about pigments that are used to dye clothes. A red dye absorbs all colors except red. You can make a dye in reverse that only absorbs red color; you have made a dye that can detect the color red if you hook it up to a neuron. When it absorbs red light, a little signal is created. So you have a cell that detects light of a certain wavelength, which we call red in English. The eyes also detects blue and yellow.

In day to day life, you do not experience monochromatic color, meaning a pure single wavelength such as lasers produce. A laser red looks fake or not as rich a red, because we are used to colors being a mixture of many wavelengths. In this aspect, color is a rich experience where there are seemingly unlimited possibilities for slight variation. A single wavelength stimulates our eye color receptors and then our brain figure out what it is most like. This is how we assign ROYGBIV to the monochromatic color spectrum. Red stimulates mostly the red receptor. Yellow stimulates yellow. Orange stimulates both red and yellow, and so forth.