Why does the sky turn red at sunset?

The views of a sunset where all the sky turns red are beautiful. But, what is the reason why the sky changes in colour at the sunset? To understand better the concepts that I will explain in this article, I leave here the article in which I talk about why the sky is blue:

Why is the sky blue?

The colours of light

Light is a type of energy that travels in form of waves through space. Each type of light (there are 7 of them in total) distinguishes from the rest by its energy, which is determined by the wavelength. The wavelength is defined as the space there is between a peak and another of the wave. In the case of visible light, which is the type of light that we can see, the wavelength goes from 380 nanometres (nm) to 780 nm. In comparison, human hair has a size of 70 000 nm.

The smallest the wavelength of the light, the higher its energy will be. Visible light, also named white light, is composed of diverse colours, which go from purple to red. Purple and blue light are the most energetic (in the framework of white light), while the red one, with a longer wavelength, is less energetic.

What happens at sunset

At sunset, (or at the sunrise), when the Sun is situated near the horizon, sunlight, unlike what takes place during the day, has to go through a much thicker layer of atmosphere before reaching our eyes. In the framework of visible light, red light is less affected than blue light by the molecules of the air. As blue light tends to be scattered more intensely through the atmosphere, this light has been scattered many times and has been deviated in many directions before reaching us. So, we can perceive much more yellow and red light.

Dust, water droplets, and pollution

The quality of the air sunlight has to go through has also an effect on the red colour of a sunset. In the case that in the atmosphere were dust particles, smoke or water droplets suspended in the air, all these elements can make more intense the colours of the sunset.

All these particles reflect light in all directions. Then, when some part of all white light is directed towards us, different quantities of short wavelengths are scattered and get lost. Therefore, we see the long wavelengths, corresponding to the orange or red.

However, the particles that pollute the atmosphere can turn a sunset red until some point. If their concentration is very elevated, as happens in the large cities, the sky is so saturated that we can no longer see the Sun clearly.

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