# B Why are there only 6 colors?

1. Oct 6, 2016

### JB321

I recently read an article written by Newton which outlined the process and results of his "crucial experiment".
From what I understand, Newton says that light can continue to be split until it reaches the basic colors and then it simply stops. What is it about these frequencies of light that makes them so special? Are there six exact frequencies that are the base of all the colors? Or are there ranges of frequencies that Newton simplified to one color? I asked my physics teacher and he responded with, "I don't know..."

JB

2. Oct 6, 2016

### blue_leaf77

Is that "6 colors" part mentioned by Newton?

3. Oct 6, 2016

### pixel

That's the likely explanation. If he used a prism, then he saw bands of color each of which contain a range of wavelengths.

I should mention, however, that the color of each wavelength can be matched by weighting of three suitable light sources. This gives rise to the CIE standard observer color matching functions which are the basis of the CIE L*a*b* color system.

Last edited: Oct 6, 2016
4. Oct 6, 2016

### Drakkith

Staff Emeritus
There are more than 6 colors. Far more. But the real idea to take away from this is that a perfect spectrum is continuous and is measured in either wavelength or frequency, not colors. You can take a slice of the spectrum and expand it until you reach the resolution limit of your system, the spectrum doesn't simply stop at some point.

Remember that color is subjective. My father is red-green colorblind, so his experience of color is very different from mine, and there are supposedly people with more than 3 types of cone cells in their eyes (the cells that allow you to see color), so their experience of color is also far different from mine.

On the other hand, the frequency/wavelength of light is not subjective. Everyone with the proper measurement device will agree on the frequency and wavelength of any sample of light.

5. Oct 6, 2016

### JB321

So why is it that if you split a ray of light several times, you will end up with an irreducible color? Say you isolate the blue section of a spectrum emitted from a prism. Why does the blue frequency not separate into different frequencies of blue? (maybe it does, but Newton seemed to say that it didn't) Is is because the frequencies are so close together that it is difficult to isolate them?

6. Oct 6, 2016

### blue_leaf77

Because the solution of the temporal part of EM wave equation is of sinusoidal form $\sin \omega t$ (or equivalently $\cos \omega t$) which has a definite frequency/wavelength called harmonics. Any EM radiation of arbitrary temporal profile can be decomposed into sum of these harmonics. That's why when a light ray is dispersed by prism or diffraction grating you get light rays each with a single indivisible frequency.

7. Oct 6, 2016

### pixel

Yes, any spectrometer will have a finite resolution.

8. Oct 6, 2016

### DaveC426913

Tetrachromats have a fourth cone that is slightly different than the usual green ones we all have. They can detect finer gradients of green than the rest of us. They might see subtleties that we don't. But that's about it.

9. Oct 6, 2016

### jbriggs444

But, in fact, you do not.

10. Oct 6, 2016

### blue_leaf77

Do not what?

11. Oct 7, 2016

### jbriggs444

You do not get a set of discrete indivisible rays.

12. Oct 7, 2016

### Khashishi

There are infinite shades of blue.

13. Oct 7, 2016

### blue_leaf77

Whether it is discrete or continuous after being dispersed depends on the spectrum of the light. Some vapour lamps have discrete spectrum, neglecting the effect of line broadening.

14. Oct 7, 2016

### Simon Peach

How can you have infinite shades of one colour? The blue light, for instance, has only a given range of wavelengths 450 - 494 nm so that range is not infinite.

15. Oct 7, 2016

### pixel

450.001, 450.002, 450.003.....or however fine you want to make it.

Are you only thinking in terms of integers?

16. Oct 8, 2016

### Simon Peach

Yes I agree that the numbers could be infinite, but the colours can't be. But thus is only nit picking really

17. Oct 8, 2016

### Drakkith

Staff Emeritus
I think Khashishi was just making the point that you can divide the portion of the spectrum corresponding to blue light into an infinite number of pieces.

18. Oct 8, 2016

### pixel

You are correct when talking about human perception of color as it takes a certain amount of color difference to be just noticeable. Instrumentally, we can resolve spectral differences finer than that.

19. Oct 8, 2016

### PeroK

How many possible frequencies are there in this range?

20. Oct 10, 2016

### DaveC426913

Just as there are infinite points between 0 and 1, so there are infinite frequencies between 450 and 494.

21. Oct 11, 2016

### sophiecentaur

The number of 'identifiably' different frequencies depends entirely on the bandwidth / signal to noise ratio, of your measurement system. The eye is not all that good as a measuring instrument. Only under stringent conditions do you actually need the 'millions of colours' that high quality colour displays use. Those 'millions' refer to the number of points in two dimensional CIE colour space and not just spectral component frequencies and implies something like 0.1% discrimination over the spectral range. As a bandwidth for measurement of frequency, that is pretty rubbishy. Frequency measurement can be done easily to 10 significant figures in some ranges of EM waves.
We specify colours in our language with much cruder accuracy. This link has a picture of the Cie Chromaticity chart, showing a very crude categorisation of areas with named colours. We can distinguish between large areas of colour (patches / contours are just detectable) when the colour space is divided into around a million smaller areas but we don't 'remember' colours with anything like that level of discrimination. (You can't take a new tie home from the shop and be sure that it will match the shirt that you left in your wardrobe but the difference will scream at you when you lay them side by side).

22. Oct 11, 2016

### Daanh

23. Oct 11, 2016

### Algr

"Color" can have totally different and even conflicting meanings if you don't define what you are measuring.

If you mix light from a red laser and a green laser, and adjust them to the correct brightnesses, you could produce light that a human would see as identical to a yellow laser. But a prisim would reveal that this light had nothing in common with an actual yellow laser. The idea that red plus green makes yellow is pure biology, not physics.

24. Oct 11, 2016

### Auto-Didact

From what I remember, Newton posited 7 not 6 colors. He chose 7 mostly for numerological reasons, i.e. purely because 7 is a holy number in Christianity.

25. Oct 11, 2016

### swampwiz

The first thing that must be understood is that "color" is a psycho-physical phenomenon, and its *perception* by humans is dependent on 2 systems: the electromagnetic wave/photon (i.e., light) receptors of the retina, and the processing of the output from those receptors by the brain. Normally, humans have these receptors (called "cones" by physiologists) that are tuned to have a maximum transfer function (i.e., how much output gets generated per input) at 3 specific wavelengths of light, that are recognized as being "red", "green" & "blue". Photons of light have a specific wavelength associated with them (i.e., in whatever relativistic reference frame an observer is in), and each photon of a specific frequency generates a specific output for each of these 3 tunings of receptors, and thus, there is a 3-D vector space for the receptors' output. The brain processes this triple coordinate as being a total "color". If the coordinates all the have same value, the brain processes it as "white" of some type (including "grey"); if the coordinates are very high in only one of those coordinates, the brain processes it as the respective color (including a darker shade).

Unless one is looking at the output from a laser device, the light that any set of receptors observe is in the form of many individual wavelengths that can be approximated as a continuous spectrum. This spectrum is in essence the "true color" of any light, with the cones & brain mapping that out to some "perceived color". To a certain level of accuracy, there are infinitely many combinations of individual wavelengths, each at some intensity, that is perceived by any particular brain as the same exact color; this is the reason why television works - a real spectrum is observed by an camera, which then produces a 3-D vector space signal that can be displayed by a display device, which a viewer would perceive in 3-D vector space as being the same as what would be perceived if viewing that original spectrum. As one might expect, this 3-D signal is best matched to human color perception by matching up with the wavelengths that correspond to the cones' maximum transfer functions, and is the reason why color is regarded as being a RGB (red-green-blue) coordinate.

There is a class of spectra called pure color (maybe it is called something else) in which the spectrum is only a single wavelength; such spectra is perceived by the brain as having a certain 3-D color coordinate value, and typically a display cannot reproduce such a spectra (except for those spectra that exactly match the output spectra of the individual display). However, these pure colors basically map to a 1-D curve within the 3-D vector space (or 2-D if intensity is normalized); there are plenty of other colors that the human color perception perceives, but these are artificial colors. A lot of these colors are close to a true color, but there is one particular section of color that is totally artificial - the colors from purple to red, which is due to the fact that the mixture of pure colors must be between these 2 colors, but yet not be along the pure color spectrum. A prism separates out light because the index of refraction is slightly different for different wavelengths; the glorious natural phenomenon of a rainbow has the same mechanism, although the particulars of geometric optics that makes it so is quite an interesting topic in its own right.

Now, as for the OP's original question of there being "6 colors", typically being in order along the pure color spectrum as violet, blue, green, yellow, orange, red, that is just perception as the brain has a hard time picking out any more of a fine gradation of colors. Now this might be my opinion, but when I look at a rainbow, I tend to notice a very large section between blue & green, that is typically know as cyan, so I would say that there really are 7 colors, not 6.

Hope this helps.