Why is the Sky Blue?


Why is the Sky Blue?

Ever wondered looking at the sky why it seems to be blue in color most of the times? The reason may be simple but surprisingly interesting. Let us know why the sky is blue in color and what is the science behind it? Usually, a clear cloudless sky in the day-time looks blue because of the air molecules scattering blue light. However, in the evening times, there is more scattering of red and yellow light. This is because, during this time, the blue light is scattered away from the line of sight.

We know that light is made up of all colors. That is, sunlight holds the entire rainbow colors – violet, indigo, blue, green, yellow, orange, and red. The air molecules in the Earth’s atmosphere interact with the rays of sunlight before reaching our eyes. These interactions cause scattering of particular color of the spectrum. Usually, the atmosphere scatters the higher-energy or high frequency color portion of the sunlight more than the low-frequency portion. This is why blue portion of the spectrum is scattered more than the red portion of the sunlight.

Rayleigh scattering

In 1859, the phenomenon of blue sky was studies by John Tyndall. He demonstrated that blue color has shorter wavelength and is scattered more strongly than the red ones. This phenomenon was explained as the Tyndall effect. Later, it was explained in detail and the phenomenon of light scattering was commonly called the Rayleigh scattering, named after the physicist Lord John Rayleigh.

It is because of this phenomenon that the Sun appears to be reddish-yellow in the sky and the other parts are colored in blue. However, this blue sky is not the same all the time. We can see different colored sky at different times of a day. When the Sun is lower in the horizon, that is, during the sunrise or sunset, the light travels through a thick atmosphere causing more scattering of light. As a result, the sky appears to be not just blue but a mixture of yellow, orange, and sometimes even green shades. During this time, the sun appears to be much redder or orangey.

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Article publié pour la première fois le 19/01/2017

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