If high frequency light is scattered, why do we see a blue sky
instead of a blue and purple or violet sky?
The sky appears blue for a combination of two reasons. Before white light reaches the Earth’s surface, the light waves collide with and bounce off of the nitrogen and oxygen atoms in the atmosphere. Different frequencies of light (in other words different colors) are scattered differently. Higher frequencies (blue and purple) are more easily scattered and thus bounce around in all different directions more than lower frequencies (red or orange) do. The scattering of high frequencies alone would cause the sky to appear blue and purple, but our eyes work better at frequencies near the middle of the spectrum (yellow and green). Since the color blue is closer to yellow or green than purple is, the sky we see appears blue.
First, the emission spectrum of the sun:
The sun is considered as basically a blackbody at a specific temperature. The peak is in the yellow range, so that would be fairly close to that 5000K curve. Note the sharp dropoff on the left after the peak (to the right is more red, to the left is more blue/violet). The violet is emitted with much less strength than blue.
Second, the sensitivity of our eyes:
Our eyes are more sensitive to blue than they are to violet. Combine the two effects and it means our eyes don’t pick up nearly as much violet light as they do blue.
If the sun were hotter, then the color of the sky might be more purple, since a hotter sun would produce a higher amount of violet light.
Why the sky turns red and orange at sunset?
“During sunrise and sunset, when the sun lies low on the horizon, the rays of sunlight must pass through almost 30 percent more area of atmosphere than they do during the day, and a higher number of larger atmospheric particles (i.e., dust, or water vapor) before they reach us. The shorter violet and blue wavelengths scatter away from our field of vision. However, the longer wavelengths of light do not scatter as much and the sky becomes filled with yellow, orange and red. Red has the longest wavelength in the visible spectrum, so when the sun lies on the horizon, it appears red. During a rainstorm, the water vapor in the air acts like a prism, separating light by the various wavelengths. This is why we see rainbows. The angle is important to seeing the separate colors, which is why when we move the rainbow appears to move.”
Stephen Corfidi, a National Oceanic and Atmospheric Administration (NOAA) meteorologist, makes an excellent point in a Q&A with National Geographic about the intensity of light and what makes a sunset or sunrise particularly “good”:
“Everything is connected. And as humans, we like to think color is concrete: ‘Oh, that’s a blue sky,’ or ‘That’s a brown table.’ But the colors you see depend on the light’s path before it got to you, how the object you are viewing reflects that light, and what your eyes are sensitive to. Absolutes don’t really exist in color perception. It’s rather disquieting when really you start thinking about it!”
So, essentially, everyone experiences a sunset in their own unique way, depending on countless tiny factors. The science as well as the individual experience is something that can be discussed for hours on end.