h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Following this discussion between my and Avenging Washcloth about the colour pink, F80678?thread=4082454

AW mentioned having arguments about magenta (being the product of blue and red light) with various 'scientific types' to quote:

"Now, I've had scientist-types argue with me that there is no such thing as a magenta wavelength, only violet. Maybe so, but they are full of poo. I use magenta light constantly as an image technician/printer and I assure you that it is a necessary and basic component of nearly every photo, computer screen image, print or color page you own."

So - what, if it exists, is the wavelength of magenta?

Clive

There is no single magenta wavelength exactly because it is a mixture of blue (about 500 nanometres) and red (about 700 nanometres) wavelengths. Avenging Washcloth's sciencey friends are correct.

This isn't to say you can't use magenta light, just that it isn't an absolute.

You might as well ask what the colour of a plate of peas and sweetcorn is. It is singular, but at the same time two undeniably different things.

B

Ah! That makes sense.

Cheers!

Clive

Ignore B, he doesn't know what he's talking about.

It's 42

I take it all back.

What nonsense!

All lights are a mixture of pure lights of different wavelengths. We only have three types of light receptors in our retinas (or less, or very infrequently more) so we can't tell what the exact composition of any particular light is. For example, we can't see the difference between a pure 'spectral yellow' and a mixture of red and green lights, but they are different.

If you want to read oodles of good stuff about this sort of thing, I recommend this page (a guide to the science of colour vision commissioned by a watercolour company for artists):

http://www.handprint.com/HP/WCL/wcolor.html

>>All lights are a mixture of pure lights of different wavelengths

Surely that's wrong - the colour of light as we see it varies with the wavelength, its simply that our eyes have three sets of sensors, each of which is sensitive to _a given range_ of wavelengths...

For all intents and purposes, unless we get down to a quantum level, there are an infinite number of colours in the visible spectrum.

Magenta is so absolute!

Just as yellow light is the midpoint between Green and Red light, so Magenta would fall between Red and Blue primary light values... but the question remains, why does the spectrum fade off to ultra-violet and infra-red rather than showing magenta?

Magenta is missing from the rainbow! I demand to know where it went!

Not at all. The spectrum really doesn't have much to do with colour-mixing. Just because we have identified red, green and blue as the three colours we can combine to make any other colour does not mean you can find them on the spectrum.

You might as well mix a pot of lavender blue paint with a pot of mint green paint, and then demand that Dulux tell you where the resultant yucky turquoise is on their colour chart.

B

That, Mu Beta, is a poopy answer.



This issue must have something to do with the way in which colors interact. I've often been amazed at the simple beauty of the correllation between the subtractive and additive methods of color mixing. Herein lies the answer to the deep mysteries of the universe. Of this, I'm sure.

Okay, here's my attempt at an explanation:

A pure (single wavelength) yellow and a pure cyan light can exist, but our eyes can't differentiate between pure yellow and a mixture of red and green. Pure yellow light would stimulate the red and green cones equally. However, because red and blue have green between them, it is impossible to have a pure colour that stimulates red and blue cones, but not green. Hence pure magenta light does not exist, but light will appear magenta if it is a mixture of red and blue light.

It is true to say that most light is a mixture of many different wavelengths, and that what we see is a kind of average based on how much our cones are stimulated by each wavelength. However, light of just one wavelength does exist, for example in laser beams.

Oh, and just to be clear, the apparent colour mixing we see is really a property of our eyes than of light itself. An animal with cones that work on different wavelengths may well see different colours where we see the same. I suspect this is the origin of the idea that cats can't see TV pictures.

...or why I struggle to see what's so interesting about pointsettias in garden centres.

It is because you are an ostrich, Clive, and therefore, cannot see red (or TV images of Chuck Norris). This is a limitation of your species.

I do understand that color is a phenomenon that we experience inside our own heads... that it does not really exist except in our perception. At least, this is what is taught.

Still, color is measured with spectrophometers... so it must exist in a quanitifiable way outside of our minds.

So, where does the magenta go in the rainbow? I've been pondering this question for some time with no complete answer. Okay, so maybe I'm crazy, but I think there is something here that science isn't explaining adequately.

Spectrophotometers* -- I'm crazy and I can't spell.

Chuck Norris's tears cure cancer.




Too bad he never cries.

Traveller in Time in a reflection of himself
"Does not fit in one rainbow, you will have to use two, the angle at wich the red and blue overlap will be what we will perceive as magenta.

Well a little more practical would be using two prisms to diffract a single slit of white light. Hold one of the prisms upside down and make the spectra overlap on a white screen.



/_| |_\
___________
White screen

"

I'll have to go buy myself two prisms, a white screen and a projector. Thank you for that suggestion.

No wait... all I need is a slit and the sun and a darkened room... and two prisms... and a sheet.... and this thermos.

I believe we have strayed from Chuck Norris to Macguyver.