3
$\begingroup$

I have read this question:

Why is the sky blue and the sun yellow?

where John Rennie says:

The only light we see directly from the Sun is the light that travels in a straight line from the Sun to our eye 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.

Now I do understand that the sky is blue due to Rayleigh scattering. The Sun appears yellowish when close to sunset because the closer it is to the horizon, light has to pass through a longer way through the atmosphere and this means more Rayleigh scattering, thus, the direct sunlight into our eyes will be weighted more towards the yellow (more blue photons scatter off).

This explains both why the sky is blue and why the direct light from the Sun is more yellowish close to the horizon.

enter image description here

But what happens after sunset? There is no direct sunlight. All Sunlight is because of scattering. How can that be then yellowish? It should all be blue because of Rayleigh scattering. After sunset, none of the photons are reaching our eyes directly. All of them are scattered off the atoms in the atmosphere. Thus, Rayleigh scattering should dominate and cause it to be just blue.

enter image description here

Just to clarify, my question is about after the sunset, when the Sun is completely below the horizon, and no photon can reach our eyes directly from the Sun, all photons are scattered through the atmosphere. Why is the sky yellow then? There are no clouds either. The question is basically if the afterglow is caused by Mie scattering and how it can dominate Rayleigh.

$\endgroup$
5
1
$\begingroup$

This phenomenon is also known as an afterglow (the opposite is called foreglow, which occurs before sunrise), which is a broad arch of whitish or pinkish sunlight in the sky that is scattered by fine particulates, like dust, suspended in the atmosphere. An afterglow may appear above the highest clouds in the hour of fading twilight or be reflected off high snowfields in mountain regions long after sunset. The particles produce a scattering effect upon the component parts of white light. The high-energy and high-frequency light is scattered out the most, while the remaining low-energy and -frequency light reaches the observer on the horizon at twilight. The backscattering of this light further turns it pinkish to reddish.

enter image description here

$\endgroup$
2
  • 1
    $\begingroup$ Thank you so much! Is this caused by Mie scattering? $\endgroup$ – Árpád Szendrei Nov 9 '20 at 17:17
  • $\begingroup$ Mie scattering happens whenever the size of the scattering particle is comparable to the wavelength of the light, however it has no upper limit on the size of the particle and, as I understand, for larger particles it should be indistinguishable from simple geometric reflection. I'm not sure if afterglow particularly happens due to Rayeigh or Mie scattering - it could be a combination of both, as the dust and fine particles vary greatly in size. $\endgroup$ – Krumuvecis Nov 9 '20 at 17:33

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.