6

The blue color of light of the sky is due to Rayleigh scattering.

But the sun itself appears yellow in color whereas the scattered sunlight itself appears blue.

Why does this happen? Should the sun then not also appear blue in color?

Qmechanic
  • 201,751
spatialdelusion
  • 444

2 Answers2

8

A thousand words

Raleigh scattering is very weak so the vast majority of the light from the Sun passes through the atmosphere without being scattered. That means when we look at the Sun we see the 99% of the light that isn't scattered, and that light has the original 5,700K colour spectrum.

The only light we see directly from the Sun is the light that travels in a straight line from the Sun to our eye - that's the horizontal yellow line in this diagram. If you consider the upper yellow line we can't see this light ray because it misses our eye. However the Rayleigh scattering due to the air scatters in all directions, so some of this scattered light reaches our eye. That means when we look away from the Sun we only see the scattered light and not the direct sunlight.

The Rayleigh scattering depends on the wavelength and blue light is scattered most. That means the light we see coming from directions away from the Sun has a spectrum weighted towards the blue. NB it isn't pure blue light. It's a spectrum of light enriched in blue compared to the direct sunlight. A spectrum of the scattered light from the blue sky is given in this answer:

Blue sky

(image from Wikipedia)

And that's why the Sun looks yellow and the sky looks blue.

John Rennie
  • 355,118
  • how is it possible that the rays coming from sun to the eye do not interact with air at all? – spatialdelusion Oct 20 '17 at 07:56
  • This is a simplification to convey the point that blue is diffused much more than yellow. –  Oct 20 '17 at 08:04
  • Then why is it not a mixture of red. orange etc if they are not scattered? – spatialdelusion Oct 20 '17 at 08:09
  • 1
    We see 99% of the light that isn't scattered. What happens to the remaining 1% of light that isn't scattered? And I find the second figure hard to interpret. Late afternoon sunlight is as direct as noon, and one latitude's noon is another latitude's late afternoon. Would be far more useful to plot it by solar zenith angle. And if direct sunlight were yellow, why is everything white on the photos taken on the moon? Doesn't that imply that the subtraction of blue light from midday sunlight is in fact significant, in spite of your 99% figure (and where does that figure come from anyway)? – gerrit Oct 20 '17 at 10:43
0

The sun is not yellow unles low above the horizon. We evolved to perceive it as white. Colour is not wave length, but a result of the brain interpreting the (usually) three stimuli in eye cones. There is a good video by Álvaro de Rújula, in the YouTube channel "Instituto de Física Teórica ITF" explainig the issue. In any case, that was already known to Goethe.

Miguel
  • 1