Why do we see the sky as blue?

We see a blue sky, because of the way the atmosphere interacts with sunlight.

White light, including sunlight, is made up of many different colors of light, each with its own corresponding wavelength.

Several different things can happen when this light encounters matter.

For instance, if sunlight passes through a transparent material, such as water, those light waves will refract, or bend, because light changes speed as it travels from one medium (air) to another (water). Prisms break up white light into its constituent colors, because different wavelengths of light refract at different angles — the colors travel at different speeds — as they pass through the prism.

Alternatively, some objects, such as mirrors, reflect light in a single direction. Other objects can cause light to scatter in many directions.

The blueness of the sky is the result of a particular type of scattering called Rayleigh scattering, which refers to the selective scattering of light off of particles that are no bigger than one-tenth the wavelength of the light.

Importantly, Rayleigh scattering is heavily dependent on the wavelength of light, with lower wavelength light being scattered most. In the lower atmosphere, tiny oxygen and nitrogen molecules scatter short-wavelength light, such blue and violet light, to a far greater degree than than long-wavelength light, such as red and yellow. In fact, the scattering of 400-nanometer light (violet) is 9.4 times greater than the scattering of 700-nm light (red).

Though the atmospheric particles scatter violet more than blue (450-nm light), the sky appears blue, because our eyes are more sensitive to blue light and because some of the violet light is absorbed in the upper atmosphere.

During sunrise or sunset, the sun’s light has to pass through more of the atmosphere to reach your eyes. Even more of the blue and violet light gets scattered, allowing the reds and yellows to shine through.

Why does the horizon color differ between sunrise and sunset?

Colors generally appear as a result of their wavelength difference and the extent of their scattering in the atmosphere. It is known that the color of the sky tends to be red at sunrise and more at sunset, but in the middle of the day the color of the sky is the bluest at all.

This is due to the fact that the sun’s rays transmit all the colors of the visible and invisible spectrum, and when they are in the bright and sunset, they are apparently farther than they are in the sky, so the red color is considered one of the colors with a large wavelength is the color that appears at the time as it is easy to penetrate the lower layers in the atmosphere aerial.

When the sun is in the middle of the sky, it is apparently closer to the earth, and therefore the blue/violet color with a short wavelength is visible as the atmosphere and the atoms stuck in it scatter it first.

It is noteworthy that fumes, pollution and dust contribute to an additional scattering of the sun’s rays, usually in favor of the red color, so the sky appears redder at sunset than at sunrise, due to the increase of the previous factors during the late hours of the day compared to the late night hours and dawn, i.e. before sunrise.

Reference :

Robert Britt (9-6-2014), “Why Is the Sky Blue?”،”

لماذا يختلف لون الأفق بين شروق الشّمس ومغيبها”،

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