opticssunvisible-light
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?
The sky does not skip over the green range of frequencies. The sky is green. Remove the scattered light from the Sun and the Moon and even the starlight, if you so wish, and you'll be left with something called airglow (check out the link, it's awesome, great pics, and nice explanation).
Because the link does such a good job explaining airglow, I'll skip the nitty gritty.
So you might be thinking, "Jim, you half-insane ceiling fan, everybody knows that the night sky is black!" Well, you're only half right. The night sky isn't black. The link above explains the science of it, but if that's not good enough, try to remember back to a time when you might have been out in the countryside. No bright city lights, just the night sky and trees. Now when you look at the horizon, can you see the trees? Yes, they're black silhouettes against the night sky. But how could you see black against black? The night sky isn't black. It's green thanks to airglow (or, if you're near a city, orange thanks to light pollution).
Stop, it's picture time. Here's an above the atmosphere view of the night sky from Wikipedia:
And one from the link I posted, just in case you didn't check it out:
See, don't be worried about green. The sky gets around to being green all the time.
Note the vertical scale on the two graphs you gave: The solar spectrum at sea level is given as an intensity (power per area), and it is very nearly flat over most of the visual range. The eye sensitivity is given as a percentage, which the wikipedia page where it is used does not explain beyond calling it "normalized" and "relative brightness sensitivity." If this percentage is akin to a quantum efficiency, the probability that any one photon gets detected, there is a very natural possible explanation for this effect: If rather than a power-based intensity we use a photon number based intensity for the solar spectrum, its maximum will be at lower photon energies (redder colors) where the same power corresponds to more photons.
Perceptions are always tricky: Neither our eyes or our brains tend to function quite the way one might naively expect, in lots of ways. Hence much of the full explanation might not depend much on (photon) physics at all.
Best Answer
Actually, The Sun outputs much more than these six wavelengths. The figure provided shows the spectrum of the Sun:
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.
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).
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.
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.