Tuesday, February 26, 2013

Mould explains the the strange properties of magenta

(Video link) Using colored flashlights, science presenter Steve Mould explains why the color magenta doesn't appear in the rainbow.

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In this optical illusion, the magenta dots switch on and off in series, producing a green afterimage on the retina. The effect is especially strong if you look at the cross in the center. Via Biotele, thanks, Damian J.

Related topic: the yellow we see on our computer monitor or our TV isn't really yellow; it's a blend of green and red, as explained in a V-Sauce video.

Previously on GurneyJourney
Mystery of Magenta


DesB said...

Thank you James. Ever a source of all things wonderful. An education.

DamianJ said...

and an optical illusion that exploits this magenta/green anomaly:

purple/green dots URL

Steve said...

Damian, thanks for the link. Impressive!

Unknown said...

So does this mean the color magenta only exists inside our brain, and just an optical illusion?

So this would mean you can not find any real life objects that are true magenta (they would be a shade purple?) and you can only see magenta when you do the trick with the light?

just trying to wrap my brain around this :)

James Gurney said...

Jens, good question, and the video may be a little misleading here. My understanding is that magenta can indeed be regarded "real" in the sense you mean it. It is considered a primary color of printing ink, developed in the 1890s. It's the "M" is CMYK, and chemical pigments can create the effect of magenta in our eyes. And for the purposes of charting the color universe, we regard it as a pure color. But as the presenter points out, it doesn't exist in pure form in the rainbow, and a certain wavelength can't be assigned to it, because it’s really a composite of red and blue. [And as a side note, it's a color that one rarely finds in nature, and that landscape painters can easily do without.]

One of the reasons color is so mystifying is that it doesn't have an objective, "real" existence like a lot of other things we experience. Our experience of it is a combination of material chemistry, idealized geometry, and neural effects inside our heads.

Keith Parker said...

So do we see the same made up color, or a unique one? Great post!

James Gurney said...

It's a lot like the way the "yellow" we seen on TV or our computer monitor is not really yellow, but a blend of green and red.
The V-sauce guy explains: http://www.youtube.com/watch?v=R3unPcJDbCc

Bruce Jensen said...

I've always found it fascinating to remind myself that the color we see from a pigment are the wavelengths that are not being absorbed by the material- it is what is reflected back. Vegetation, which is most commonly green, is absorbing red and blue light- leaving only the green to be reflected back. Magenta of course is just performing the opposite.

great video!

Scorchfield said...

Another questions, why two quarks have name like „charmed” and „strange”? Why - in this time, our brain (like Murray Gell-Mann brain in 69) discover new directions of „aromas” of colour?

David Briggs said...

Hi James!

Here's the sad truth about the V-sauce video. The guy contrasts the yellow monitor screen with a banana, which he says is a "real" yellow because it reflects yellow wavelengths. It does, but it also reflects a lot of red and green wavelengths, and most of the yellow we see is an additive mixture of this red and green light, like on the screen. I don't have a spectrum for a banana, but there are reflectance spectra for some yellow paints here:

As for the screen colour, the green lights actually do contain a component of yellow wavelengths (see Fig. 4.1.2 on my first page on additive mixing). So really, both banana and monitor give off a mixture of red, yellow and green wavelengths, the difference is only the the monitor light spectrum is more spiky, with somewhat less energy in the yellow part of the spectrum. Kind of kills a good story, I know!

In any case, rather start than talking about things "looking" yellow but not "being" yellow, I think it is better to just say that yellow perceptions need not require the actual yellow wavelengths of the spectrum.

The magenta video is a slightly odd presentation of the old Young-Helmholtz model (odd because magenta in that model is a mix of red and blue "fundamental sensations", and wouldn't seem to present any particular "mystery"). In the modern zone theory, ALL hue perceptions are created in the brain based (for isolated lights, anyway) on the relative cone responses. The colour magenta is no more or less real than any other hue perception, it's just that the stimulus that creates it needs to contain a mixture of wavelengths. I explain this a little more towards the end of a long page on the history of the primary colours here:


James Gurney said...

Thanks, Briggsy for taking the time to explain those important qualifications. The truth of color perception is always more complex than it appears, and not as "neat" as we'd like it to be, making it hard to convey in a short presentation.

Anonymous said...

James, at first I thought you were going to be talking about wood trims, or something green and slimy:-)

Seriously though, you always seem to bring ideas to your blog that we don't always associate directly with artwork. We get caught up in the painting to the point where we don't always explore the "whys" or related topics.

I appreciate the way you wander off into other fields that allow us to tie it all together with our art and help us broaden our perspectives. I don't say "thank you" often enough for all you share. Thank you.

James Gurney said...

Marsha, you're welcome. That's so nice of you to say. I enjoy doing it because I learn, too.

Kessie said...


It's been raining and rainbow-y outside, so I went out and tried to spot magenta. It's right underneath the purple band, like it's trying to shade back into red.

Sam Nielson said...

It is crazy that we perceive the violet end of the spectrum as rolling back toward red, because there is absolutely no relationship there. Yet somehow, we evolved to loop what is just a snippet of a larger spectrum of light into something that appears to cycle.

Thanks for the explanation and link, very interesting!

David Briggs said...

I keep coming across people who have picked up completely wrong ideas about colour vision from the Mould and Vsauce videos, so I've belatedly added a comment on both on Youtube. As they will soon be swallowed up among the thousands of other comments there I'd like to also post them here.

Magenta video (Mould): This video is an exposition of the original Young-Helmholtz model of colour vision, which was a revolutionary step in our understanding of colour, but is far from current science. We now know that the so called "red cone" (L cone) is most sensitive to the yellow part of the spectrum. And according to the modern zone model of colour vision, the brain does not get "messages" directly from the three cones at all, but creates red/green and yellow/blue signals based indirectly on comparisons of the cone respones. So the brain "makes up" all colours in exactly the same way as it "makes up" magenta.

Yellow video (Vsauce): Vsauce: A lemon is "subtractively yellow", "it absorbs all visible wavelengths of light except for yellow light, which it reflects onto my retina". No it doesn't:
A lemon reflects most of the red, orange, yellow and green light that falls on it, and most of its yellow colour comes from the additive mixture of the red and green wavelengths, like on your screen. The same is true of all bright yellow objects:

David Briggs said...

Me again. I've just posted a much fuller version of those comments on my website here: