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My physics book says that six colors can be distinctly seen in white light: red, orange, yellow, green, blue, and violet. Does solar light only use these six wavelengths and mix them additively, or does it use a range of colors from red to violet?

And what effect does the scattering of blue light from the sky have on the solar spectrum?

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    $\begingroup$ Are you asking about "white light" from the sun or about white light in general? You only need three different frequencies of light (red, green, blue) to make light that looks white to us. $\endgroup$ – Brandon Enright May 31 '13 at 15:23
  • $\begingroup$ I'm not sure. My Physics book doesn't specify if it's talking about the sun or not. It only says, "white light . . ." $\endgroup$ – Daniel Pendergast May 31 '13 at 15:25
  • $\begingroup$ In that case I think your question is basically a duplicate of physics.stackexchange.com/q/24263 $\endgroup$ – Brandon Enright May 31 '13 at 15:27
  • $\begingroup$ In that case, I'll reword it to refer to white light of the sun. $\endgroup$ – Daniel Pendergast May 31 '13 at 15:29
  • $\begingroup$ Alright related questions physics.stackexchange.com/q/48399 and physics.stackexchange.com/q/61912 $\endgroup$ – Brandon Enright May 31 '13 at 15:36
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Actually, The Sun outputs much more than these six wavelengths. The figure provided shows the spectrum of the Sun:

enter image description here

As you can see, the Sun outputs light along a continuous curve at all visible wavelengths. The combination of which appears white to our eyes as a byproduct of having evolved in orbit of this star.

The image also shows the absorption effects of the atmosphere. If you specifically discuss the blue scattering from the atmosphere, the following figure is a nice depiction.

enter image description here

As you can see, the blue scattering leaves only the more red visible wavelengths, which makes sunlight appear more yellow around midday and more red near dawn/dusk.

As for why we can simulate white light perfectly without having to reproduce the solar spectrum; that has everything to do with how humans perceive colour. The human eye has special cone cells capable of perceiving colour. As shown below, there are only three true colours we can see; red, gree, and blue (with more green cells than the others again due to the solar spectrum).

enter image description here

It is by using a combination of these three colours that we interpret all possible visible wavelengths. Our brains interpret the relative strengths by which each different colour cone is stimulated and assigns a different colour to each combination. The next figure shows the sensitivity ranges of these cones.

enter image description here

The three types of cones are sensitive over all visible wavelengths but are stimulated differently by each wavelength. Thus, we can interpret a nice teal when a 500nm wave stimulates the cones appropriately, or we can simulate this for human eyes (and this is what we do with things like computer screens) by combining certain levels of 400, 535, and 700nm (thanks to Peter Shor for the correction) lights to stimulate the cones by the same ratio as the 500nm light. Being the same ratio, our brain is tricked into seeing it as teal, but the summed light itself is not actually teal. In the same way, a combination of the six colours you mentioned would look white to us, but to make a true white, the Sun must output across all wavelengths in the visible spectrum at varying amounts.

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  • $\begingroup$ If you want to combine three wavelengths to get colors, it's actually much better to use wavelengths of 400nm, 535nm, and 700nm. If you use 445nm, 535nm and 575nm, I don't think you can get anything redder than pure 575nm wavelength light, which is a yellowish-orange. See this website. $\endgroup$ – Peter Shor Aug 6 '13 at 16:38
  • $\begingroup$ @PeterShor This is true, but I think you couldn't see the forest for the trees $\endgroup$ – Jim Aug 6 '13 at 16:44

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