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When we look at the sky, it's blue because blue light has shorter wavelength, so it scatters more energy; and at sunset we see a red sky because most of the blue color is scattered out. What I don’t understand is that, at sunset and daytime, the blue light is scattered out more than the red light, the color and wavelength won't change with time. If we see red sky, it is because blue light is scattered out more. Why is the sky blue during daytime, when it is scattered out more as well.

I see lot of explanations that it’s because much blue lights are scattered out due to the longer distance to our eyes during sunset than daytime, but during daytime the blue lights is still scattered out more than red light when lights come to our eyes, so why the sky is blue instead of red?

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  • $\begingroup$ Hello, your post discusses the different colors at sunset and DAYTIME. But your title asks about the difference between sunset and SUNRISE. You should edit your post or your title to make them match. $\endgroup$
    – James
    Mar 9, 2023 at 16:22
  • $\begingroup$ Possible duplicate: physics.stackexchange.com/questions/722675/… $\endgroup$
    – g s
    Mar 9, 2023 at 17:16
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    $\begingroup$ why the sky is blue instead of red?...because blue light is scattered the most(lower wavelengths). Since blue light is scattered the most, it does not appear to the eyes that blue light is coming from the sun(to a great extent but not all), instead we perceive blue light coming from the sky. This is the exact reason why the sky would look black if there were no atmosphere(in such case, only direct light from sun reaches the eyes). $\endgroup$
    – khaxan
    Mar 10, 2023 at 8:23

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Our eyes are adjusted to the sunlight in daytime, which means that the sun itself during the day appears white. Light with higher frequencies appears blue, light with lower frequencies appears red. Once a white light goes through the atmosphere, blue light scatters more, so the atmosphere -- that is the piece of sky around the sun -- appears blue. (it scatters in all directions, but some of the scattered blue light goes into our eyes)

When we talk about evenings, the light has to pass through a thicker layer of the atmosphere. This means that more of blue light scatters on the way from the sun to our eyes (some part still scatters into our eyes, but some is scattered into other directions). The more blue light scatters the less goes trough. This is why, comparing to daytime, the sun itself appears more red. Still, the sky around the sun should be somewhat bluer then the sun, as during the day. Roughly, the light that scatters in upper layers of the atmosphere is primarily blue. But to reach you eyes it goes trough other layers and scatters there, so when it reaches you a large portion of blue is scattered and it becomes more red. The light that scatters in lower layers is already red, as only red light from the sun reaches there. So, eventually, the sky is also red. Of course, to make it accurate, you should rather solve a differential equation considering scattering at each infinitesimal layer of the atmosphere.

The key thing that needs to be highlighted here is the following. The color of the sun is defined by the dominating frequency of the light emitted by the sun minus the light scattered by the atmosphere. Instead, the color of the sky is defined by the dominating frequency of the sunlight which is scattered by the atmosphere. So, these colors are formed in a sort of complementary fashion.

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  • $\begingroup$ Thanks for your answer. if the blue light scatter more by thicker atmosphere during sunset, and that causes less blue light go through our eyes, so the sky appears red. But during daytime blue light still scatter more than red light, so there should be less blue light going to our eyes compared to the red light. So why the sky is blue during daytime? $\endgroup$
    – Xiang Li
    Mar 9, 2023 at 15:21
  • $\begingroup$ "if the blue light scatter more by thicker atmosphere during sunset, and that causes less blue light go through our eyes, so the sky appears red" -> not the sky appears red, but first of all the sun appears red. Look at the last paragraph of my reply. The atmosphere subtracts blue from the sun (and makes it more red) and the atmosphere itself shines blue with this subtracted light. In the evening the atmosphere layer for the sunlight to pass is thicker. So the reddening effect on the sun is stronger. The sky in the evening is still more blue than the sun itself, as during the day $\endgroup$
    – Dr.Yoma
    Mar 9, 2023 at 16:46
  • $\begingroup$ please note that dawn also has a lot of red, 'red fingered dawn" en.wiktionary.org/wiki/… $\endgroup$
    – anna v
    Mar 9, 2023 at 16:58
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    $\begingroup$ @XiangLi, see this image. You can notice that the sky still has a faint blue colour. Also, the sun appears mostly red, that is because red light from the sun directly reaches our eyes(most of it). $\endgroup$
    – khaxan
    Mar 10, 2023 at 8:27
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During the day, with the Sun high in the sky, most of the blue sky that we see is due to single scattering, even near the horizon. Photons from the Sun interact with particles in the atmosphere, scatter and arrive at our eyes. Most of that light is blue.

The reason that single scattering dominates is that the optical depth to visible light photons is less than 1. By that I mean the chance of a photon from the Sun interacting with the atmosphere before hitting the ground is low, when the Sun is high in the sky - typically 30%-40% for blue photons and much lower for red. Similarly, light that is scattered tends to reach us from a point in the atmosphere where the optical depth to us is still less than 1. i.e. The scattered light comes straight to us without being scattered again.

When the Sun is low in the sky, its light must travel through a much thicker layer of atmosphere to arrive in our vicinity. The optical depth to blue photons is much greater than 1. Very few photons can arrive directly and those that do are red. Where scattering events take place in the general direction of the Sun is still at a position separated from us by a thick layer of atmosphere, so these scattered photons are also attenuated in such a way as to favour red light over blue.

The net result is that the sky is redder in the direction of the Sun near sunset. The blue photons are taken out of the beam and end up in space or hitting the ground far away.

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