The fascinating physical phenomena behind the reddish beauty of sunsets

The fascinating physical phenomena behind the reddish beauty of sunsets

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“The light that reaches us from the Sun when it is on the horizon does not have any blue light anymore, it practically has only red light”, explained the astrophysicist Fernando Jauregui.

It is one of nature’s most captivating shows.

A sunset that spreads and radiates in the sky its intense luminosity and scarlet tones.

But what are the principles of physics that are hidden behind these beautiful sunsets?

What phenomena cause the horizon to look reddish or does our star appear flattened? And what is the curious green ray?

Fernando Jauregui, an astrophysicist at the Pamplona Planetarium in northern Spain, explains the interactions that give life to these spectacular sunsets and remembers how to protect the view by admiring them.

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1. Reddish horizon

Formation of birds in a reddish sky
Copyright of the GETTY IMAGES image
Image caption The distance that sunlight is passing through the atmosphere is much greater when the Sun is setting than when it is at noon.

There are several ways that sunlight has to interact with the atmosphere as it passes through it.

It is said in physics that the atmosphere is a slightly dispersive medium, because sunlight has all colors but these do not behave in the same way when they pass through the Earth’s atmosphere.

The colors m to s blue is more easily spread the colors m ost red . That is why by day the sky looks blue, because the blue light that comes in the rays of the Sun is scattered by the molecules of the air forming that blue sky that comes to us from all directions.

When you say ” spread ” this means that the light ” strikes ” against the particles in the atm or Sfera and desv í to?

Let’s say that the term “colliding is a vulgarization of what really happens.” In physics, we usually say that light interacts with molecules and that interaction is more effective when light is bluer than when light is redder.

Why?

Because the molecules have a certain size – the molecules of nitrogen, oxygen, carbon dioxide – and it is a size similar to the wavelength of the blue color and when that happens, the light and the molecules interact.

Seen from the outside, it is as if it collides with the blue color against a molecule and goes off to the other side.

Why do we then see the reddish horizon?

The distance that sunlight is passing through the atmosphere is much greater when the Sun is setting than at midday.

The light that comes to us from the Sun when it is on the horizon does not have any blue light anymore, no yellow light, no green light, practically only has red light. That’s why we see more red.

2. Flattened sun

Sun that seems flattened in a sunset
Copyright of the GETTY IMAGES image
Image caption The atmosphere is as if it were a lens and diverts the sun’s rays.

This is a different phenomenon, which has nothing to do with the interaction with the atmosphere but with refraction .

The atmosphere at that moment is doing as if it were a lens, it is deflecting the Sun’s rays in such a way that they reach our eyes in different directions than they have before reaching the atmosphere. And that’s why we see the sun deformed, flattened.

It is a refractive effect, not a recreation effect.

3. Dimmed sun

Man riding a bicycle with a sunset background
Copyright of the GETTY IMAGES image
Image caption The atmosphere absorbs part of the light of the stars or of the Sun in this case and to our eyes less light arrives when it is lower on the horizon.

It is another effect that in astronomy is well known that is called atmospheric extinction. It means that as the light is scattered by the atmosphere, all the light from the star does not reach our detector and less light comes as the Sun is closer to the horizon.

Let ‘s say the atm ósfera absorbs some of the light from the stars or the ol in this case and our eyes less light reaches the lower is on the horizon, that has spanned more air layer of the atmosphere and therefore has been attenuated to a greater degree.

4. The green ray

It is a very beautiful phenomenon that occurs in certain atmospheric conditions only.

Sunset with a green flash, the so-called green ray of light
Copyright of the PEKKA PARVIAINEN / SCIENCE PHOTO LIBRARY IMAGE
Image caption Sometimes the coincidence occurs that the only light of the Sun, the last ray of light that reaches the position in which we are, is the one corresponding to the green light.
Green flash in a sunset
Copyright of the image STEPHEN & DONNA O’MEARA / SCIENCE PHOTO LIBRARY
Image caption On the top of a mountain it is easier to see the green ray than if you are in a valley.

What exactly happens is that when the Sun has already set, sometimes the coincidence occurs that the only light of the Sun, the last ray of light that reaches the position in which we are, corresponds to the green light That’s why you see a green flash.

It is a phenomenon that lasts very little, a few seconds and that occurs when certain atmospheric circumstances occur.

It depends on the state of the atmosphere and the position in which you find yourself. For example, if you are on the top of a mountain and you have a clean, very distant horizon, it is easier to see the green ray than if you are in a valley.

What precautions must be taken to see these phenomena ?

You always have to be very careful to look at the Sun.

The luminosity of the solar disk is so enormous that it can damage our retina and the sunburn is irreversible and very serious.

Galileo Galilei
Image caption Galileo looked at the Sun only when there was fog or when the star was already very, very dark, and still lost his sight at the end of his years.

They say that Galileo , who used a telescope that he himself made and could not project the image of the Sun but had to look through the telescope, at the end of his days had practically lost his sight.

He looked only when there was fog or when the sun was already very, very dark, and still he lost his sight.

Hands of a woman in yoga position during a sunset
Copyright of the GETTY IMAGES image
Image caption You always have to look at the Sun very carefully.

That is why whenever there is an eclipse of the Sun astronomers say that “do not look at the Sun directly”.

In the case of sunsets, when the sun is already very, very low and the atmosphere has fog and the attenuation is large, it allows you to look at it with the naked eye, but always with the naked eye, never with instruments such as telescopes or binoculars , because instruments make loupe effect.

Returning to the reddish color. The sunsets remind us then that the atmosphere is full of vibrating particles.

The atmosphere is always around us and as we do not see it, it seems that it does not exist.

But the atmosphere is absolutely fundamental for us and is quite present. It is so present that airplanes that weigh a lot of tons are held in the air.

Sunset
Copyright of the GETTY IMAGES image
Image caption The atmosphere is full of particles and they are all moving because they have a temperature.

So really there are many particles and they are all moving because they have a temperature. Thanks to that there is that atmosphere we are here, because it gives us adequate pressure, oxygen to be able to breathe, and we have evolved with these conditions.

We have to be very grateful to the atmosphere and we would do well to take care of it. We have to treat it better because the truth is that if we change it we are altering the physical conditions that allow us to live.

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