High School Why does light diffract into only seven colours?

[hutchphd]

and only the rich kids got to see in 64 colors..

That way you could see in the dark and locate people by "seeing" how their body temperature is higher than ambient.

Here's one news article about that, but it's about research conducted by the Chinese:

https://www.slashgear.com/night-vision-eyedrops-nanoparticles-research-infrared-vision-28567893/

I think the distinction between "night vision" (which is Near Infrared (NIR) often actively illuminated usually silicon detected devices) and thermal imaging FIR devices needs to be emphasized here. Only the thermal imaging devices will see the glowing enemy combatant in the dark field but these are very different from usual silicon optics or eyeballs.
I once had to explain this to a client who had already spent a fair sum of money trying to develop a silicon CCD-based imaging thermometer. Lack of understanding can be expensive, and I felt like the Grinch as I invoiced him for my services.

 

[LCSphysicist]

?

 

[LCSphysicist]

What was that i had just read

 

[LCSphysicist]

Please, everybody knows there are 96 colors. (Used to be only 64)

View attachment 289324

Mine was just twelve

 

[phinds]

Mine was just twelve

Lucky guy. Mine was one. It was coal. It was a lump of coal. We had to color everything black. If there hadn't been lines in the coloring book then when we got done it would have been just all black. And sometimes you couldn't see the lines and it WAS all black. You young whippersnappers with your fancy "colors". Bah humbug.

 

[Vanadium 50]

Coal? You had coal? When I was a kid your so-called coal was still dinosaurs.

 

[DaveC426913]

Loogxury.

 

[louis_slicka]

Hi all thanks for the detailed and varied replies to my very flawed question! I realized as soon as I posted that there are an almost infinite variations on the basic seven colours. It's like asking how long is a piece of string

 

[hutchphd]

We have managed to play with that piece of string for quite a while! Good fun for all..

 

[hilbert2]

I think the distinction between "night vision" (which is Near Infrared (NIR) often actively illuminated usually silicon detected devices) and thermal imaging FIR devices needs to be emphasized here. Only the thermal imaging devices will see the glowing enemy combatant in the dark field but these are very different from usual silicon optics or eyeballs.
I once had to explain this to a client who had already spent a fair sum of money trying to develop a silicon CCD-based imaging thermometer. Lack of understanding can be expensive, and I felt like the Grinch as I invoiced him for my services.

A good point. You'd have to detect wave lengths over 1 micrometer to have temperature differences near room temperature be visible, and the chemical properties of surfaces would also affect the appearance at the same time.

 

[sysprog]

One of the 64 colors in the big Crayola crayon box was a pinkish color called 'Flesh'. I asked my Dad (who as an attorney and law professor did a lot of civil rights work) wasn't that a bit racist? $-$ my 'negro' friends didn't have close to that skin color $-$ Dad told me (I was maybe age 7) to not worry about it $-$ that was for grown people like him and Mom to deal with $-$ I should just keep on being my friends' friend.
:;

 

[Vanadium 50]

It is now "peach".

And no human being ever had skin that color.

 

[DaveC426913]

It is now "peach".

And no human being ever had skin that color.

Well true, but Macaroni and Cheese was never that colour either...
Nor Outer Space, that colour...

http://www.jennyscrayoncollection.com/2017/10/complete-list-of-current-crayola-crayon.html

 

[sophiecentaur]

Well true, but Macaroni and Cheese was never that colou

The colour named Indigo is often placed well inside the CIE graph, rather than on the spectral locus. It's a pretty dodgy part of our colour perception range around there.

 

[Anachronist]

Fun fact: not all colors are found in the rainbow. There's no magenta, and no cyan.

No, actually there is cyan in all rainbows, as well as white light split by a prism. But there is no magenta.

 

[sophiecentaur]

No, actually there is cyan in all rainbows

The problem with that statement is that Cyan is a mix of blue and green primaries (It can also be considered as -Red but that system is based on primaries. Primaries are not spectral wavelengths and can only add together to produce resulting colours on a straight line on the CIE chart. That means it cannot actually lay on the (very much curved) Spectral locus. A rainbow is not actually a very pure spectrum; it's very de-saturated because there's a lot of white in there. Basically, a rainbow is not a very good example of the spectrum of sunlight. The colours can often look impressive and vivid but nothing like as good as what you get in a dark room with a prism.

 

[DaveC426913]

The problem with that statement is that Cyan is a mix of blue and green primaries.

Maybe on a monitor, but there's a frequency of light between blue and green that correlates with cyan (at least, allowing for saturation, as you say).

The problem with magenta is that there is no single frequency, since magenta is a perceived mix of red and blue. If you took the frequency between red and blue, you'd get a green.

 

[sophiecentaur]

Maybe on a monitor, but there's a frequency of light between blue and green that correlates with cyan (at least, allowing for saturation, as you say).

The problem with magenta is that there is no single frequency, since magenta is a perceived mix of red and blue. If you took the frequency between red and blue, you'd get a green.

Yeah I can’t really disagree. I was being mischievous but no colour you can make with primaries is actually spectral. Lowering the saturation involves adding a third primary.

nothing is real with colours.

 

[MysticWizard]

https://en.wikipedia.org/wiki/Color_wheel

 

[Baluncore]

After all we have only three distinct eye pigments and yet I swear there are seven colors when I look at the rainbow.

There are a couple of ways to demonstrate why it might be seven.

We detect three different colours. Red, Green & Violet, that we sometimes call Blue.
That gives us a three bit binary number with 2³ = 8 possible combinatorial states.
But black is zero, so only 2³ - 1 = 7 colours remain.
If we are colour blind to one colour we see 2² - 1 = 3 colours.
If we had an extra detection pigment we would see 2⁴ - 1 = 15 colours.

But the rainbow is different because photons are ordered by wavelength, so in the rainbow detected Red cannot be adjacent to and mixed with Violet. Our Red and Green detectors are significantly overlapped which makes it all the more complicated. Anyhow, in the rainbow we see;
Red = Red.
Orange = Red, with a touch of green. (R+G with R>G).
Yellow = Green, with a touch of red. (R+G with R<G).
Green = Green.
Blue = Green, with a touch of violet.
Indigo = Violet, with a touch of green.
Violet = Violet.
So that gives us seven named and identifiable colours in a rainbow.

I am not surprised that most people agree on the presence and the spelling of 7 colours, even if some misspell the word colour.

 

[hutchphd]

So that gives us seven named and identifiable colours in a rainbow.

Very interesting. I was wandering down those same (3-bit) channels myself while staring at the CIE chart of colors (please note the compactness of the absent silent "u")
In particular the results of the various pigment deficiencies seem congruent with this idea. But it is not clear exactly how these spectra are rendered. For instance
https://www.researchgate.net/figure/Top-color-spectrum-as-seen-with-normal-vision-Next-6-rows-the-three-main-types-of_fig2_342092723

 

[Baluncore]

Very interesting.

Is gold an orthogonal colour ?
Apparently it can be found at the end(s) of a rainbow, perpendicular to the colours.

 

[Merlin3189]

There are a couple of ways to demonstrate why it might be seven.

We detect three different colours. Red, Green & Violet, that we sometimes call Blue.
That gives us a three bit binary number with 2³ = 8 possible combinatorial states.
But black is zero, so only 2³ - 1 = 7 colours remain.
If we are colour blind to one colour we see 2² - 1 = 3 colours.
If we had an extra detection pigment we would see 2⁴ - 1 = 15 colours.

If 000 is black, would not 111 be white, leaving 6 colours ? Which is what most people see.

I suspect a protanope might still see six colours in a rainbow, as do normal people, but not necessarily the same ones.
That is based on the notion that 7±2 is the number of categories we can recognise. If asked to discriminate between small sections of the rainbow, I think everyone could divide it into many more than 7.

 

[Baluncore]

If 000 is black, would not 111 be white, leaving 6 colours ?

Yes. But the physiological cone detectors get weighed and balanced dynamically by the brain, so white is coloured. We then use rod detected brightness to determine the difference between dark and light.
https://en.wikipedia.org/wiki/Color_vision#Non-spectral_colors

https://en.wikipedia.org/wiki/Color_vision#/media/File:Cone-fundamentals-with-srgb-spectrum.svg
If you scan across the resolved spectrum in that image, and classify situations based on the three cone detectors, you can look for clearly differentiated colours. But then all colour balances are variable, based on the background and recent bleaching of the three different cone detectors.

Maybe it comes down to how many colours a child needs to recognise and name correctly, before the family can communicate and survive to pass on the language with the names for the different colours.

 

[JeffJo]

Late to the party, I know. But in skimming the thread I didn't see the right answer, and did see much that is wrong. (And speaks poorly of Newton.)

First, Newton separated the colors of the spectrum (not the rainbow) into seven categories of color (not discrete colors).

• The rainbow has different colors. The spectrum's are all single wavelengths of light, and the rainbow's are composites of one tight group of wavelengths that is very bright, and less dim contributions from all the others toward the red end of the spectrum. In fact, the rainbow continues all the way to its center as gray.

• He chose seven categories of color, not seven discrete colors.
• "Blue" in Newton's day was more like sky-blue; that is, the ninth and tenth bands in that picture.
• What you probably think of a "true" blue, as opposed to cyan or sky blue, is what he called Indigo. It is the color of indigo dyes, as made popular by "blue jeans" which get their classic color from that dye.
• The relationship to music, and the planets, was a more a result of his categorization, than a driving force.

The story goes that Newton had poor eyesight. So he asked a friend to draw divisions between the colors produced by his spectrum. Friend chose five categories: Red, Yellow, Green, (sky?) Blue, and Violet. What Newton noticed was that two of these categories (Red and Blue) covered roughly a 50% a wider extent along the spectrum than the other three (Yellow, Green, and Violet). He also knew that in the Pentatonic Minor Scale (he used D minor: D, F, G, A, C, D+), two of the intervals (D to F and A to C) covered a 50% wider pitch range than the other three (F to G, G to A, and C to D+). And wouldn't you know it, the ordering was the same as the colors! There also is a link to the planets, but I'm not sure what that was.

So Newton added two categories, orange and indigo, to what Friend had drawn. (And yes, he ended up with a Dorian scale, not the Minor scale). Here's his representation:

 

[sysprog]

The 'visible light spectrum' is the entire range; the individual frequencies are termed 'spectral colors'; subranges within the spectrum are called 'spectra'.

 

[hutchphd]

First, Newton separated the colors of the spectrum (not the rainbow) into seven categories of color (not discrete colors).

• The rainbow has different colors. The spectrum's are all single wavelengths of light, and the rainbow's are composites of one tight group of wavelengths that is very bright, and less dim contributions from all the others toward the red end of the spectrum. In fact, the rainbow continues all the way to its center as gray.

Please provide a reference for this claim. It does not comport with my understanding of either light or Newton.

 

[Anachronist]

As others have noted, "indigo" generally isn't used nowadays. Instead of the 7-color spectrum ROYGBIV, I'd remove "I" (indigo) and add "C" (cyan) for ROYGCBV. Still seven.

It also makes more sense to break down the color categories into six groups, corresponding to each color receptor type in our eyes (red, green, blue) and the colors in between them (yellow=red+green, cyan=green+blue, magenta=blue+red). That's basically how we perceive colors. Violet or purple is basically magenta skewed a bit toward blue at lower luminance, and orange is basically yellow skewed toward the red.

 

[JeffJo]

Please provide a reference for this claim. It does not comport with my understanding of either light or Newton.

What part of the claim? There was a link to Newton's history, which is well known so I have to assume that isn't what you meant. So it must be the rainbows?

I showed you the picture of it. It came from here, one of the leading references on rainbows:
https://www.atoptics.co.uk/rainbows/primcol.htm

Or you can follow the math at:
http://www.trishock.com/academic/rainbows.shtml

Here's my graph of the equations in that reference, for red light:

Consider a great circle of the drop. The horizontal axis considers all of the light that hits the drop along that great circle, as a function of the distance each ray would have passed from the center of the drop if it had not been there. So the energy density along this line is constant. The blue lines, if I recall correctly, show how much deflects within a 0.1 degree range. As this range approaches zero, the deflected energy density within the range approaches infinity. Like this:

In the color bars I showed, the bright areas show the diameter of the sun.

 

[JeffJo]

As others have noted, "indigo" generally isn't used nowadays. Instead of the 7-color spectrum ROYGBIV, I'd remove "I" (indigo) and add "C" (cyan) for ROYGCBV. Still seven.

Those are pretty much the categories Newton used. But what you call Cyan, he called Blue. What you call Blue, he called Indigo (as in "blue jeans" which are indigo). Only the border between them is unclear.

It also makes more sense to break down the color categories into six groups, corresponding to each color receptor type in our eyes (red, green, blue) and the colors in between them (yellow=red+green, cyan=green+blue, magenta=blue+red). That's basically how we perceive colors. Violet or purple is basically magenta skewed a bit toward blue at lower luminance, and orange is basically yellow skewed toward the red.

That is indeed one way. It wasn't what Newton did.