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At daytime the Sun's light is yellowish if not near white. Why when the Sun starts to go down that it's light turn more red. I don't think the earth's rotation is so rapid to cause a red shift.

Why isn't that effect as clear, at sunrise?

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This is due to Rayleigh scattering, i.e. the shorter wavelengths (those near the violet end of the visible spectrum) are more deflected by dust particles than the longer wavelengths (those near the red end of the visible spectrum).

When the sun is near the horizon, it the path of the light through the atmosphere (and in particular through layers which have a higher concentration of dust particles) is longer, thus the scattering away of the non-red components is more pronounced.

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    $\begingroup$ Thank you very much. Another matter I can't understand, why does the sun appear white at sunrise unlike during sunset? $\endgroup$ – Ray Feb 18 '14 at 22:30
  • $\begingroup$ I've seen many red sunrises. Are you sure you start watching the sun in the morning at the same altitude (height above the horizon) as you experience it red in the evening ? Is the region west of you more dusty than the area east of you (e.g. land on the west, sea on the east) ? Is air pollution in your area stronger in the evening than in the morning ? $\endgroup$ – Andre Holzner Feb 19 '14 at 8:51
  • $\begingroup$ You are probably right I will believe you on this one, since the sun rises behind some buildings in my place and sets in the sea (where I can see it go all the way down). However, why I had doubts that its not as red at sunrise is that it has something to do with redshift. But my guess is that it's irrelevant, am I correct? $\endgroup$ – Ray Feb 19 '14 at 9:27
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    $\begingroup$ yes, redshift from Doppler effects are negligible (40'000 km / day is about $1.5 \cdot 10^{-6}$ of the speed of light). If the redshift were responsible for the sunlight turning red in the evening one would expect it to be affected equally by blue shift in the morning... $\endgroup$ – Andre Holzner Feb 19 '14 at 17:33
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I don't get this. Surely if the sky is blue in the daytime because the blue light is preferentially scattered, colouring the whole sky blue, like white light in fog, this effect would be even more pronounced at sunset, when there is more atmosphere for the light to pass through and therefore more particles to concentrate the blue light.

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  • $\begingroup$ Hello, and welcome to Physics SE. Look around, and please take the tour. As for your answer, I'm not sure how to square scattering of blue light during the day with a concentration of blue light towards sunset. As the path length through the scattering medium (air) increases, doesn't this just mean there is less blue making it through to you? $\endgroup$ – Jon Custer Sep 28 '15 at 19:37

protected by Qmechanic May 20 '16 at 0:47

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