Why is the color of the clear sky blue

If you look at the sky from space on a summer day, it is black and sunlight glows white. Why is the color of the clear sky blue

When viewed from the ground, the colors appear different: the sky is an intense blue, with the sun projecting a warm yellow light.

Blue sky with colorful sunshine

The reason why the sky appears blue when viewed from Earth is the nature of sunlight. Sunlight is made up of individual rays of light that travel in waves.

When you look at all the rays of light together, the light appears white. However, if light is deflected by a prism, for example, individual spectral colors such as red, orange, yellow, green, violet or blue appear. Hence the sun’s rays are made up of vibrant colors.

Rayleigh phenomenon explains blue skies

The sun’s rays enter the earth’s atmosphere while traveling to the earth. These are invisible gas molecules, mainly nitrogen and oxygen. When the sun’s rays hit these tiny particles, they get deflected or scattered. Since the wavelength of each color is different, the scattering is also different.

When the Sun is high in the sky, the path that light has to pass through the atmosphere is relatively small. Blue light is mostly diffused – the sky appears blue. This phenomenon is also called Rayleigh scattering.

Englishman John William Strutt, 3rd Baron Rayleigh discovered the physical theory that causes blue skies in the 19th century.

Red sunlight creates color at sunset?

At sunrise or sunset, the sky will display colors different from blue during the day. Variations of red tones replace blue, and the Sun, which appears yellow during the day, also appears red.

This is because the Sun’s rays are longer in the atmosphere at dawn or dusk because the Sun is shorter – mainly red light is scattered. Why: After a short distance, the molecules block short-wave blue light; Only long-wave red rays reach Earth. It is shown as sunrise or sunset.

Image by Giani Pralea from Pixabay

Experiment with Flashlight: Sun and Blue Sky

By pouring milk into a tall glass of clear water, you can imitate the scattering of light from the sky. The fat molecules in milk, like the molecules in the atmosphere, scatter light from a torch. Light appears blue, the light source produces a yellow glow like the Sun.

If the sky is at its most beautiful blue when the Sun shines, it is because the sunlight is being deflected by the smallest particles in our atmosphere.

Simply put, sunlight is made up of many single rays of light that travel like a wave. When we look at all these rays of light together, the light appears white to us.

As light strikes a prism or raindrops at a certain angle, we can see different light rays, the so-called spectral colors: for example in a rainbow composed of red, orange, yellow, green, blue and violet. is invented.

Each of these spectral colors has a different wavelength of light, that is, there is a different distance between two successive wavelengths. For example, red light has a long wavelength, while blue light has a very short wavelength.

Blue sky during the day

When the Sun is high in the sky, the path for light to pass through Earth’s atmosphere is relatively short. Earth’s atmosphere is largely composed of nitrogen and oxygen molecules.

Light rays collide with these tiny particles in the air and are deflected by them, or more accurately: they are scattered. Short-wave blue light is scattered more strongly by air molecules than long-wave red light. Since it is mainly blue light that is reflected off the smallest particles of air, we see a clear, cloudless sky blue.

Rayleigh scattering gives the sky its color

This physical principle was discovered in the 19th century by the Englishman John William Strutt, 3rd Baron Rayleigh. In his honor, the phenomenon that gives us blue skies is called Rayleigh scattering.

Not only does it describe in concrete terms what happens when sunlight is scattered by air molecules, but it also applies to all types of electromagnetic radiation in general and to all particles that emit this radiation. are smaller than the wavelength of air.

The power with which these particles emit absorbed radiation is inversely proportional to the fourth power of the wavelength of this radiation: the shorter the wavelength of the radiation, the more it spreads.

Red at sunset, gray in bad weather

At dusk or dawn, however, the conditions are completely different: at sunrise or sunset the sun is low in the sky, and the path the light must travel before reaching the viewer is much longer.

The short-wave blue light is then intercepted by the molecules after a short distance and we get only the long-wave red part.

But as we know, the sky doesn’t always show us its most beautiful colors, it can also appear gray and cloudy. This is always a sign of dusty or moist air, not to mention constant cloud cover.

Sunlight on relatively large particles of dust and water is not broken down into its spectral colors, but is reflected directly like a mirror, and thus the sky appears grey-white.

Other planetary sky

But what about the other planets? Is the sky blue or red? It depends entirely on the atmosphere of the celestial body, more precisely on its density and composition.

For example, the Moon has no atmosphere: the sky is always black and sunlight hits the surface non-stop, so it appears crisp white.

Mars, on the other hand, has a very thin atmosphere, consisting mainly of carbon dioxide and small particles of rust (iron(III) oxide).

The NASA probe that explores Mars is therefore always looking at the yellow-red sky. The atmosphere of Venus consists mainly of carbon dioxide, but it is very dense.

In addition, it is surrounded by a 20 km thick cloud layer, which consists largely of sulfuric acid. It is difficult for sunlight to penetrate these thick layers.

However, color images from the Soviet Venus probe show that the sky is visible.

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