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Comment by phkahler | Hacker News Reader

Why is violet in the rainbow not a very blue color? I would think it only activates the blue cones. 405nm is a nifty color.

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Because the cone isn't really a "blue" cone, and neither is the "red" one. The curves overlap in complex ways. A pure violet photon also slightly stimulates the long wavelength cone.

That's why red+blue=purple feels a bit like violet. It creates a similar double firing.

(And why red plus green gives an even more accurate yellow. The long and medium cones have a lot of overlap.)

This is a common misconception, but the sensitivity of L cones ("red" cones) increases monotonically until about 570nm (monochromatic yellow), so violet light stimulates L cones the least out of all visible wavelengths of light. Magenta light, a mixture of red and blue wavelengths, stimulates L cones far more than violet light. See Wikipedia's LMS responsivity plot[1] or the cone fundamental tables from the Color & Vision Research Laboratory at [2].

I think the misconception comes from plots of XYZ color matching functions[3]. The X color matching function indeed has a local maximum in the short wavelengths, but X doesn't represent L cone stimulation; it's a mathematically derived curve used to define the XYZ color space, which is a linear transform of LMS color space selected for useful mathematical properties.

[1]: https://en.wikipedia.org/wiki/LMS_color_space#/media/File:Co...

[2]: http://www.cvrl.org/

[3]: https://en.wikipedia.org/wiki/CIE_1931_color_space#/media/Fi...

It is technically the bluest color possible. What we perceive as true blue is different, and the brain has the weird imaginary magenta gradient between blue and red to confuse.

Meganta isn’t imaginary, it’s just non-spectral.

It's imagined only in our minds, it fits the definition better than anything else.

First of all, all colors are imagined only in our minds.

Second, the term imaginary color already exists, and it refers to a specific thing [0], and the colors on the line of purple are not one of them. What you are describing is a non-spectral color. They exist in day to day life and in nature, they simply do not have an associated wavelength.

[0] https://en.wikipedia.org/wiki/Impossible_color

What exactly are you trying to prove? The gradient between red and blue (magentas) are the only fully saturated colors that we can perceive, which aren't part of the electromagnetic spectrum. That's fantastic. Do you want to waste your life arguing about nothing instead of enjoying the miracles of nature?

Blue light looks different from violet light, because blue light activates M cones ("green" cones) more than violet light does.

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