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I would like to ask about the phenomenon, which always occurs near sunrise/sunset.

enter image description here

If I could be more precise. When Sun goes down its strong reflection is to be spotted usually at trees, but also other objects. It's expressed well when dealing with long-distance photography, especially when the solar disk is enormous in comparison to a distant tree. It looks like the trees (or other objects) are optically "on fire". It's best visible shortly after sunset (literally 1-2 seconds after the sun's disk disappears beyond the horizon).

enter image description here

Is anyone able to explain this phenomenon or even better - post some links, and sources where I could read more about it?

UPDATE:

Another example is presented here: enter image description here

The first solar beams just go through the northern slopes of the hill located nearly 200km away (black arrow), whereas both low-level clouds nearby, as well as the snow blown by northern winds, are significantly illuminated (all the green arrows). The Sun is literally just about to rise with the 1st contact already made. The entire solar disk is just beneath the horizon, but all the features located nearby literally transmit direct sunlight. How could this phenomenon be explained? Does it indicate the same effect described?

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  • $\begingroup$ I expect this is a duplicate, so I won't write a full answer. The distance to the horizon is further at high elevations than it is at ground level, so it takes slightly longer for the treetops to "see" the sunset. See siranah.de/html/sail040o.htm $\endgroup$
    – PM 2Ring
    Nov 21, 2022 at 22:55
  • $\begingroup$ Yes, they can "see" the sunset, but it looks like a quite serious bit of sunlight is held on their branches. This is what I am asking about. $\endgroup$
    – Geographos
    Nov 22, 2022 at 7:43

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I believe this is an optical effect that has long interested physicists, astronomers, and poets, as discussed John Hardwick's 2020 article "Sunrise on the pines" in the journal Applied Optics. Mentioned by Shakespeare (Richard II, Act iii, sc. 2, line 1450), the first known scientific description was in an 1832 letter from Louis Necker to David Brewster.

Great Irish physicist John Tyndall "was ambiguous as to the origin of the phenomenon likening it to the diffractive glare around the sun but also explicitly referring to reflection from the pine needles’ smooth surfaces." Others believed it to be specifically due to diffraction from the narrow pine needles, and Jacques Babinet cited it as example of the optical diffraction theorem that we now know as Babinet's principle. Marcel Minnaert also described it as a diffractive phenomena in his book "The nature of light & colour in the open air". But François Folie was skeptical. Any physicist or astronomer wandering the Alps around sunset or sunrise seems to have been at high risk of being nerd sniped by the phenomenon.

According to Hardwick, it has not been resolved how much of the effect is diffraction and how much is due to low angle specular reflection. The problem with diffraction around pine needles is that they are not thin enough to produce the angular dispersion of up to several degrees that has sometimes been reported. The problem with simple models of specular reflection is the opposite, the reflection wouldn't fall off as fast as observed. I would guess both processes contribute, depending on the details of what is doing the diffracting/scattering/reflecting. You mention observing the effect for objects other than trees, and such observations could be very helpful for disentangling what is going on.

Necker's original observation was on the Salève near Geneva, so this might be an interesting after-hours research project for some physicist at CERN.

Note: If you don't have access to the paywalled Hardwick article, you can get the contact information for the author by clicking on "Author Information" on the article landing page.

UPDATE:

The newly added (14 Nov 2023) photo is harder to analyze because it may include both the effects discussed above and the Novaya Zemlya effect where the Sun appears to rise early or set late because atmospheric refraction. This can produce a bright line on the horizon as shown in this YouTube video and in many online images. My guess is that this is dominant in the new photo, but to disentangle the effects, you'd need more images taken at sunrise and sunset under different atmospheric conditions and seasons. The Novaya Zemlya effect depends on atmospheric conditions while the diffraction/reflection effects depend on the vegetation at the top of the distant hills.

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The reason for the red glowing of the trees and the snow is the same as for why the sky appears red during sunset/sunrise and blue during the day: It's Rayleigh scattering. (You'll find plenty of literature on the term 'Rayleigh scattering', sky, sunset, etc.)

On its way through the atmosphere, light is scattered at the small molecules and atoms (water, dust, air itself) within the air. This scattering is highly dependent on the frequency of the light: The scattering cross section (i. e. the probability that the scattering takes place if the path of the beam is near the atom or molecule) is proportional to the fourth power of the frequency of the light!!

$ \sigma_s \propto f^4 $

That means: The probability whether the light is scattered or not very strongly depends on its frequency. High frequencies (blue light) have a much higher probability to be scattered on their way through the atmosphere than lower frequencies (red light).

During sunset/sunrise, there are less photons from the sun reaching your eyes via those trees than during full daylight. And from these few, only those photons which can make their straight path from the trees to your eyes you will register. The probability that a photon can make this large straight path without being scattered away is much higher for red than for blue light.

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