Why isn't the sky purple?

I’ve read Cecil on why the sky is blue. link here
I understand that blue light scatters more than red light because it has a higher frequency but why then would the sky be purple or violet? They have even a higher frequency.

its all a trick of the eye
essentially enough red and green are scattered to make the eye think its seeing blue

It was once, in 1999, for just 3½ minutes.

Sorry, I was dreaming when I wrote that. :smiley:

The wonders of color vision. “Purple”, they always taught us, is anything along that line on the CIE Chromaticity diagram that joins the two ends of the spectral locus (the line corresponding to pure spectral colors). Purple is therefore a mixture, and has no single associated frequency.

You might be confusing it with (or replacing ) Indigo, the color Newton interposed between blue and violet, and which most people would be hard-pressed to define today. I certainly can’t. One of the things that used to be dyed with indigo were blue jeans, which are undoubtedly blue to me. I strongly suspect that the persistence of the name in the spectrum is so that a.) We can continue to have seven colors; and b.) so that Roy G. Biv will have a pronounceable last name.
As for violet, it’s a recognized color, but the response of the human eye is falling off pretty swiftly there, so it doesn’t make a big contribution to the color you see. Despite what scm1001 says, I wouldn’t say it’s a “trick” of the eye. Your eye interprets mixtures of colors as a single color, and can do so in many ways. You can see any color bounded by that Chromaticity diagram, and when you perform the calculation of multiplying the strengt of the source, the scattering coefficient, and the response of the eye and summing over all the wavelengths of light, you end up with a color in that light blue region on the map. I actually performed this calculation, pretty much by hand, in my days in grad school. see Warren J. Smith’s Modern Optical Engineering, or a similar text (or wenbsite) for the gory details.

CIE Chromaticity Diagram (the straight line at the bottom right is the locus of “purple”):



What CalMeacham said, with a little more:

Actually, if you look at that chromaticity diagram, you will see that the limiting color as wavelength drops is little different from the blue you see with more visible wavelengths. But your ability to detect those shorter wavelengths falls off very quickly, so they contribute less and less to the overall sensation.

Also, the sky is scattering all wavelengths, though less of the longer ones. There are still enough greenish rays getting through to move your perception somewhat away from that curly little corner on the CIE diagram.

I’ve seen a green sky before a big storm. How did this happen?

After a little googling, I got this old article.

Slightly off topic here – concerning the purple/violet thing.

My understanding was that purple is a mixture of red and blue, and that violet is light with a frequency slightly higher than blue.

My assumption (no cite) was that violet appears in our perception to be much the same as purple basically because the red-sensitive cells in our retina are stimulated by the high frequency violet light.
Our eyes are sensitive to reds with wavelengths as long as 750nm. Sensitivity to violet drops off at around 380nm: that is, roughly double the frequency. I had always assumed that at these wavelengths the red-sensitive cells were simply picking up the second resonance. (Badly worded, but you know what I mean.)

So here’s the question: Is my assumption in any way correct?

I just looked through a prism spectroscope. The short end of the spectrum looks different from blue, and it looks like purple or violet but not as red as magenta - at least, as I understand the colors.

The red and blue sensors have peaks around 442 nm, but for the red sensor the bigger peak is at 598 nm. Those don’t differ by a factor of 2. The sensors get their sensitivity through dyes, and they use different dyes. I don’t know any reason the red dye ought to have a resonance where the blue dye peaks. It’s not like the sensors are grating spectroscopes.

Just a WAG, but does the sun output any more blue light than violet? Does that contribute to the effect?

Well, I’ve had it told to me that it is because God is a Penn State fan.

This contradicts post #4 which basically says that enough of the other colours get through that the mixture of all the colours is the blue of the sky.

But scm1001’s link in post #2 does show a second peak in red sensitivity in the blue region of the spectrum.

>does the sun output any more blue light than violet?

Yes. It looks pretty much like a blackbody peaking longer than blue, and the falloff at shorter wavelengths is steeper than at longer. There are also narrow absorption lines, but they take up little of the total, so we can neglect them. The Sun outputs a boatload of light at all wavelengths, but much more so in blue than in violet.

Oddly enough, the peak wavelength for the sun is yellow! :cool: