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
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.
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