How come light in the Violet spectrum looks like a mixture of Red and Blue, even though they are of different wavelength? [duplicate]

Question

How come pure monocromatic light in the violet spectrum is perceived by us as light that is a superposition of 2 other, different wavelength light.

is there some scattering process in the eye that causes a both an filtration of one photon (lets say Red) and a shift in the wavelength of the Blue (like Compton/Raman scattering)?

Edit: I'm looking for at least a electrodynamical\quantum mechanical explanation. how come purple is "measured" when the wavefunction consists of 2 particles of different wavelength

Answer 1

No, there's no physical process that results in this. The process is biological — the way color vision works. What humans see from a mixture of red and blue is not violet, at least not the maximally-saturated violet that you get from e.g. 405 nm laser light. Instead it's a desaturated violet, somewhere between the line of purples and the white point.

Below is the CIE 1931 chromaticity diagram. On the top curve, marked with blue numbers, are the spectral colors — the ones resulting from monochromatic light. All the interior is the mixtures of multiple wavelengths. Addition of two colors works as choosing a point between the two colors you want to mix, depending on what proportion each color is taken in. If you try this with spectral blue like 460 nm and red like 640 nm, you'll not get a point on the spectral curve. So you'll not get the same color as from a 405 nm or any other purely violet light source.

Answer 2

To add to Ruslan's answer, remember that there are three different color-sensitive cones in our retinas (four for a small portion of the population). What we perceive as a color is based on the relative signal from each cone-type. Since there's overlap in sensitivity ranges, with different sensitivity values there, a lot of 'fun stuff' happpens in our brain to interpret the inputs.
So, roughly speaking, the relative signals from all the cones turns out to be the same when either specific red & specific blue photons are incident OR when specific violet photons are incident.

Answer 3

To add just a little more to the existing answers here, the characteristic of your eye which makes it interpret color mixtures as a single color is called metamerism and is a standard topic in the world of physiological optics.