# Why isn't the sky purple? **URL:** \ **Category:** Factual Questions\ **Created:** [September 7, 2007, 1:48am UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325 "2007-09-07T01:48:21Z")\ **Posts on this page:** 15\ **Page:** 1
**Author:** ![solkoe](https://avatars.discourse-cdn.com/v4/letter/s/919ad9/32.png) [@solkoe](https://boards.straightdope.com/u/solkoe)\ **Post date:** [September 7, 2007, 1:48am UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/1 "2007-09-07T01:48:21Z")
I’ve read Cecil on why the sky is blue. [link here](http://www.straightdope.com/classics/a810828b.html) 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. ---
**Author:** ![scm1001](https://avatars.discourse-cdn.com/v4/letter/s/c4cdca/32.png) [@scm1001](https://boards.straightdope.com/u/scm1001)\ **Post date:** [September 7, 2007, 5:16am UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/2 "2007-09-07T05:16:10Z")
its all a trick of the eye [http://www.math.ucr.edu/home/baez/physics/General/BlueSky/blue\_sky.html](http://www.math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html) essentially enough red and green are scattered to make the eye think its seeing blue ---
**Author:** ![HMS\_Irruncible](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/hms_irruncible/32/7394_2.png) [@HMS\_Irruncible](https://boards.straightdope.com/u/HMS_Irruncible)\ **Post date:** [September 7, 2007, 12:43pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/3 "2007-09-07T12:43:18Z")
[QUOTE=solkoe] 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? [/QUOTE] It was once, in 1999, for just 3½ minutes. Sorry, I was dreaming when I wrote that. 😃 ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [September 7, 2007, 9:02pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/4 "2007-09-07T21:02:22Z")
> [@](#): > > 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. 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”): > **[This is Chromaticity Diagram Applet](https://www.cs.rit.edu/~ncs/color/a_chroma.html)** > **[Chromaticity](https://en.wikipedia.org/wiki/Chromaticity)** > > Chromaticity is an objective specification of the quality of a color regardless of its luminance. Chromaticity consists of two independent parameters, often specified as hue (h) and colorfulness (s), where the latter is alternatively called saturation, chroma, intensity, or excitation purity. This number of parameters follows from trichromacy of vision of most humans, which is assumed by most models in color science. In color science, the white point of an illuminant or of a display is a neutral ... . ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 7, 2007, 9:14pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/5 "2007-09-07T21:14:27Z")
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. ---
**Author:** ![solkoe](https://avatars.discourse-cdn.com/v4/letter/s/919ad9/32.png) [@solkoe](https://boards.straightdope.com/u/solkoe)\ **Post date:** [September 7, 2007, 9:59pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/6 "2007-09-07T21:59:19Z")
I’ve seen a green sky before a big storm. How did this happen? ---
**Author:** ![Der\_Trihs](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/der_trihs/32/233_2.png) [@Der\_Trihs](https://boards.straightdope.com/u/Der_Trihs)\ **Post date:** [September 8, 2007, 2:04pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/7 "2007-09-08T14:04:52Z")
[QUOTE=solkoe] I’ve seen a green sky before a big storm. How did this happen? [/QUOTE] After a little googling, I got [this](http://www.gi.alaska.edu/ScienceForum/ASF2/262.html) old article. > [@](#): > > Scattered light that comes to the eye from parcels of air located different distances away are richest in various colors. Whereas the scattered light coming to the eye from the overhead sky is richest in blue light, that which comes from 20 miles (30 kilometers) away is richest in green light. Light from 25 miles away is richest in yellow, that from 30 miles away is richest in orange, and that from a greater distance is richest in red. Consequently, the sky near the rising or setting sun is orange and red since the brightest scattered light is coming from air many miles from the observer. > > Hence if a heavy cloud deck covers an observer’s sky out to a distance of about 20 miles away, the light the observer will see will have green color because the clouds are shielding out the blue light scattered from nearby. If the cloud deck expands out to successively cover the sky to distances 25, 30 and more miles away, the observer will see the color of light below the clouds change to yellow, orange and then more reddish. ---
**Author:** ![j\_sum1](https://avatars.discourse-cdn.com/v4/letter/j/8baadc/32.png) [@j\_sum1](https://boards.straightdope.com/u/j_sum1)\ **Post date:** [September 9, 2007, 8:36am UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/8 "2007-09-09T08:36:08Z")
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? ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 9, 2007, 11:49am UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/9 "2007-09-09T11:49:37Z")
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. ---
**Author:** ![chrisk](https://avatars.discourse-cdn.com/v4/letter/c/6de8d8/32.png) [@chrisk](https://boards.straightdope.com/u/chrisk)\ **Post date:** [September 9, 2007, 12:03pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/10 "2007-09-09T12:03:33Z")
Just a WAG, but does the sun output any more blue light than violet? Does that contribute to the effect? ---
**Author:** ![Billdo](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/billdo/32/315_2.png) [@Billdo](https://boards.straightdope.com/u/Billdo)\ **Post date:** [September 9, 2007, 12:27pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/11 "2007-09-09T12:27:25Z")
Well, I’ve had it told to me that it is because God is a Penn State fan. ---
**Author:** ![solkoe](https://avatars.discourse-cdn.com/v4/letter/s/919ad9/32.png) [@solkoe](https://boards.straightdope.com/u/solkoe)\ **Post date:** [September 9, 2007, 1:20pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/12 "2007-09-09T13:20:36Z")
[QUOTE=j\_sum1] 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? [/QUOTE] 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. ---
**Author:** ![flex727](https://avatars.discourse-cdn.com/v4/letter/f/258eb7/32.png) [@flex727](https://boards.straightdope.com/u/flex727)\ **Post date:** [September 9, 2007, 2:58pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/13 "2007-09-09T14:58:12Z")
[QUOTE=solkoe] 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. [/QUOTE] But **scm1001** ’s link in post #2 _does_ show a second peak in red sensitivity in the blue region of the spectrum. ---
**Author:** ![Napier](https://avatars.discourse-cdn.com/v4/letter/n/ce73a5/32.png) [@Napier](https://boards.straightdope.com/u/Napier)\ **Post date:** [September 9, 2007, 8:47pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/14 "2007-09-09T20:47:07Z")
\>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. ---
**Author:** ![flex727](https://avatars.discourse-cdn.com/v4/letter/f/258eb7/32.png) [@flex727](https://boards.straightdope.com/u/flex727)\ **Post date:** [September 10, 2007, 2:31pm UTC](https://boards.straightdope.com/t/why-isnt-the-sky-purple/418325/15 "2007-09-10T14:31:24Z")
[QUOTE=Napier] \>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. [/QUOTE] Oddly enough, the peak wavelength for the sun is yellow! 🆒