We all know the Sun as the big yellow disk in the sky, providing us with light and warmth. However if we look at the Sun at sunrise, sunset or even through a cloud, it suddenly turns red or orange and around noon it seems to be white. If we stop to think about it, then we realise that this is quite odd, what causes these changes, does it have a real color?
To begin to understand this, we, for once, have to start with the answer. Namely that the Sun is always white. This is not at all so strange if we think about it, as if it would not have been white, we would not have all the colors of the rainbow.
Looking back at the article “why is snow white“, we can recall that white light is actually a combination of all the colors and that colored light is just one wavelength of light.
The best example for this are the yellow street lights you see around highways and in tunnels, these lights only emit yellow light and cause everything around you to be either yellow or black. Therefore because we see all colors during the day, this light must be white. We can confirm this by looking at a picture of the Sun, taken from the International Space Station (ISS) (See top picture). This, however does not explain why the Sun looks yellow or red, so lets get to that.
This effect is caused by our atmosphere, the air and water molecules in the sky scatter and absorb blue and violet light a lot easier than the other colors, spreading it across the sky, coloring it blue. The other light however, can pass through relatively unaffected depending on the amount of atmosphere (airmass) it has to pass. We call this process Rayleigh scattering.
This concept is illustrated very nicely by the picture above. Here you see the white light of the sun being partially scattered, leaving only a part of the colors that we began with. However, it should be noted that the light that the Sun emits is not in equal quantities. The colors in the middle of the spectrum (yellow and green) are emitted more strongly than red, blue and violet, which explains why the sun looks yellow while its remaining rays contain red, orange, yellow and green.
Now when the sun is low on the sky, its light has to pass through a lot more air to reach your eyes.
Giving the air the chance to scatter not only the blue and violet light, but also a large part of the green and yellow rays. This leaves us with red and orange, explaining the color of the sun at sunset and sunrise.
Basically, the farther the light has to travel through the atmosphere, the less of it will reach your directly. Which also explains why the sun seems red when seen through clouds, as they do the same as the atmosphere only more efficiently.
In conclusion, sunlight is white, but the color that we perceive it to be depends on how far it has to travel through the atmosphere. The same goes for the sky, the lower the Sun, the more of the scattered light ends up in the atmosphere, coloring it from blue to red.
Another example of reighley scattering are lasers, where you can see the beam of blue and green ones, while red beams are not visible without aid.