# 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.