Rayleigh scattering and red sky

You sit on the sand and look to the horizon to watch the sunset. The sky seems poetically to change color, turning orange and ocher. It can even be reddish or purple. Of course, you just enjoy the show. But what if you wonder why this is happening? Okay, we’re probably going to break the spell now when we talk to you about Rayleigh scattering. It has little to do with poetry and a lot to do with physics.

What a poetic picture ... but here we are going to talk about Rayleigh scattering, a physical phenomenon.
What a poetic picture … but here we are going to talk about Rayleigh scattering, a physical phenomenon.

First, we need to understand what light is made of. It has all the colors of the visible spectrum. Namely red, orange, yellow, green, blue, indigo and purple. The phenomenon we are talking about is related to the optical properties of sunlight as it passes through the earth’s atmosphere. This is how the astronomer Edward Bloomer of the Royal Museums in Greenwich, London explains.

“It has to do with how sunlight is scattered. That doesn’t work consistently, ”explains Bloomer. Each color has a different wavelength and therefore the different shades are due. For example, purple has the shortest wavelength. While red has the longest.

The next step is to understand our atmosphere. For example the gas layers that surround our planet and make life possible. When sunlight falls through these layers of different densities, it bends and breaks down. It’s like going through a prism.

As the sun sets or rises, its rays collide at certain angles with the upper layers of the atmosphere. And then the “magic” arises. When the rays enter, the blue wavelengths are split and reflected. “When the sun approaches the horizon, the blues and greens dissipate. We had that orange-red sheen, “says Bloomer.

The same phenomenon explains why sunsets sometimes turn purple.
The same phenomenon explains why sunsets sometimes turn purple.

Shorter wave light (purple and blue) scatters more than longer wave light (orange and red). The result is a fascinating color display in the sky. “Clouds of dust and smoke can affect the view of the sky,” says the astronomer. The atmosphere can contain more particles that reflect light, depending on weather conditions.

Incidentally, the Rayleigh scattering phenomenon also explains why the sky tends to look bluer at noon. The sun is at the highest point in the sky. Its light passes intact through the atmosphere, it is absorbed as it is and the predominant visible color is blue.

Of course, things can change depending on the weather.

When it rains while the sun is shining, each drop of water breaks the light into its different wavelengths. The result of this refraction distributes all the colors in the atmosphere. And what appears in the sky? A rainbow.

We know all of this thanks to a 19th century physicist, John William Strutt, third Baron of Rayleigh. He was known as Lord Rayleigh. He spent much of his time observing sunlight and the atmosphere, and he was the first to explain why the sky is blue.

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