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> many people have never actually seen the colour "violet" which is a single wavelength of visible light

The violet seen in a rainbow (in nature, not a photo) is legit single wavelength violet. Same with the rainbows created from shining white light through a prism.

It's true that you don't really get to see it in isolation very often though. Maybe some flowers, birds, or butterflies? Or maybe the purple glow you get from UV lights?

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.

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.

> The purple colour we see is formed from a mixture of red and blue, whether it's something in nature, screen displaying or printing.

Well if it’s on an RGB screen, or printed with CMYK inks then it’s not ‘real’ violet, but there must be plenty of natural and artificial pigments that are actually reflecting violet light and not blue + red light. I imagine any pure compound would be doing this. E.g cobalt phosphate (aka cobalt violet).

You could tell by illuminating a sample with different light sources. See metameric failure:

https://en.wikipedia.org/wiki/Metamerism_(color)#Metameric_f...

> Because there are very few things that reflect this wavelength in reality.

You mean few things that reflect only this wavelength? Because I would think anything white would reflect this wavelength just like any other.

Yes, I meant reflecting only this wavelength. Thanks.

Violet is a true wavelength, and does occur in nature.

Magenta, formed by mixing red and blue, does not exist in nature. For that reason, "magic pink" (full-brightness magenta, #ff00ff) is often used as a transparency color when the image format does not support an alpha channel (e.g., sprite sheets, Winamp skins).

It's not true to say that mixtures of red and blue 'do not exist in nature'. Fuchsia petals really are that color. All you need is a substance that preferentially absorbs green wavelengths but reflects reds and blues.

What 'does not exist in nature' is a single wavelength that produces the equivalent stimulation of your L, M and S cone cells as a mixture of red and blue light does.

But most of what we see in nature is not single wavelength light - it's broad spectrum white light reflecting off things with absorption spectra.

The reason stuff looks so weird under certain LED lights or pure sodium light is that the source light isn't broad spectrum - it's missing wavelengths already - so the way it interacts with absorption spectra is unintuitive. Something that looks blue under white light should still look blue under blue light - but a blue LED might just be emitting blue frequencies that the object absorbs, so it looks black instead.

You can make the difference between a single wavelength color and a composite color which looks the same, by looking at objects nearby.

If they are of one of the composite colors, they should appear in their natural hue

Else they will just appear darker

How did you spread laser light over larger area?

The idea I'm having right now is reflecting it off of the rough side of aluminum foil.

I remember we just simply shone at a white wall.

You did send a specific wavelength to your retina, but that wasn't violet. Because violet is a construct by your brain.

Color is not a property of wavelength. There's nothing special about photons wiggling in the 380 to 750nm range.

In general it's not necessary to be this pendatic, but given the topic here, I think it's important to realize this. It takes a while because we are so good at projecting our internal experience outward.

Remember the blue / black dress?

In my personal conception, violet is the kind of colour at the lower edge of the rainbow, which is a single wavelength. And purple is what the brain constructs. However, of course, the names of the colours are themselves vague.

Hmm if you talk to a colorist violet and purple are 2 different colors one more on the red and the other more on the blue. That’s still the construct of 2 wavelength colors. So a made up color of our brain that doesn’t exist.

"Violet" is a spectral color, which means that it is a color formed by a single wavelength of light. And it is a member of the rainbow (the spectrum).

"Purple" is a mixture of red and blue.

Maybe that's a language issue, because purple and violet are color names around here.

And as such, they are both a construct of the brain, as any other colors, like... white.

What we label as "violet wavelength" is only a narrow projection of our experience outward. Case in point, we don't have such colorful (eh) names for other EM wavelength.

I say narrow because you could take this pure laser and change th surrounding and you will inevitably perceive it differently, even though the power and wavelength are the same.

> did send a specific wavelength to your retina, but that wasn't violet.

It was, by definition

> Color is not a property of wavelength.

Sure, it's a label

> There's nothing special about photons wiggling in the 380 to 750nm range.

There is - they activate different receptors your brain relies on, hence leading to a distinct (from other wavelengths) sensation

The waves aren't inherently special, your retina is.

What if we were sensitive to the 200 to 500nm range? What would be blue, violet and red then?

Our eyes and brain are the one constructing what we perceive as color. It doesn't exists outside of us.

Here's good article on the subject: https://anthonywaichulis.com/regarding-perception-photograph...

>What if we were sensitive to the 200 to 500nm range?

https://www.youtube.com/watch?v=A-RfHC91Ewc

Violet is a real wavelength, below blue on the spectrum. Where it becomes invisible to the human eye, it starts getting called ultraviolet.

Magenta and purples are constructs by the brain, as you mention.

No, they are all constructed, including blue.

If I shine some wavelength to your eyeball and you say "it looks blue", but then I change the surrounding and now it looks white, I don't think you would conclude that the original wavelength is blue.

We have a many examples like this, which prescribe that vision is not at all an accurate wavelength measurement device.