Why is the Sunset Red?

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Sunsets often produce beautiful images. The sky and clouds are painted in different colors – red, pink, yellow, pale blue and even violet. Obviously, all this beauty is possible due to the fact that the setting sun changes its color from bright yellow to orange and even red. But why is this happening?

In fact, the physics behind the process are very simple.

As you know, an atmosphere surrounds the Earth. The atmosphere consists mainly of two gases – nitrogen and oxygen. In addition, argon, water vapor, and carbon dioxide are present in small amounts. All this exists in the form of molecules randomly flying around us and colliding with each other.

What happens to light when it passes through Earth’s atmosphere?

The light of the Sun, before reaching our eyes, must first overcome 150 million kilometers – the distance from the Sun to the Earth (and there is no problem – the light does this in only 8 minutes), and then pass through the atmosphere. And here, sunlight begins to have problems.

The light that the sun emits is almost white. Well, maybe a little yellowish. What does it mean? This means that in a beam of sunlight there is light of various wavelengths – from violet (whose wavelength is approximately 400 nm) to red (760 nm). Sunlight is mostly wavelengths of green and yellow light, merging and mixing with violet, blue, orange and red. Thus, appears white.

When sun rays hit the atmosphere, air molecules randomly fly here and there. These molecules are very small, but the wavelength of visible light is also small. Consequently, part of the light will be scattered in different directions and will not reach our eyes. And maybe it will, but having re-reflected from some other molecules.

Is the light of different wavelengths scattered equally?

It turn out that no, it doesn’t. The shorter the wavelength, the more difficult it is for the light to avoid colliding with the molecules encountered in its path. This is understandable – it is easier for long-wave light to go around an obstacle than for short-wave light.

Therefore, blue and violet light coming from the sun will be better scattered in the sky than orange and red.

Now look at the sky during the day. What color is it? Of course, blue, unless cloudy. And what color is the sun? Most likely yellow! Why? Because it lacks the very blue and blue light that was taken away and scattered by the Earth’s atmosphere to become white.

What happens at sunset?

During sunset (or sunrise), when the sun is low in the sky, sunlight passess through a much greater thickness of atmosphere than it does during the day. This means that sunlight will lose even more shortwave light until it reaches us. It comes to the point that the atmosphere scatters even the yellow and green light of the sun in sufficient quantities, coloring the sky in a variety of shades. As a result, only the longest-wave red light reaches us directly from the Sun. And our daytime star itself at sunset is not at all bright. Indeed – the atmosphere takes away most of the sun’s shine.

Why is one sunset different from another?

It depends on the state of the atmosphere at the horizon on a particular evening. Small particles suspended in the air enhance the effect. For example, in the steppes, sunsets in summer are often very beautiful due to fine dust in the air. Dust easily takes away almost all the light from the Sun, since its particles are much larger than air molecules. If you look at the Sun through the dust or smoke of a fire, it will always be red. The largest, ordinary dust or mist, like water vapor, will scatter all the light of the Sun evenly. At sea, the particles of salt often play the role of dust, which create the Tyndall effect.

By the way, the scattering of light by large particles – the Tyndall effect – can be easily reproduced at home. Take a glass, pour water and dissolve a little ordinary soap in it. Then pass through the glass the light of a white flashlight. It will be yellowish at the exit.

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