How many times have you looked at the vast sky above and thought why is it blue in colour? Or why is the sky sometimes red or orange during sunset and sunrise? Where do these colours come from? Well, here’s everything you need to know about why the sky is blue and why it changes colours.
As white light passes through our atmosphere, tiny air molecules cause it to scatter. The scattering caused by these tiny air molecules, also known as Rayleigh scattering, increases as the wavelength of the white light decreases. As you must have studied in your Physics textbooks, violet and blue light have the shortest wavelengths and red light has the longest. Hence, blue light is scattered more than red light and the sky appears blue during the day.
The same phenomenon of scattering is responsible for the sky turning red, orange, and pink at the time of sunset. Typically, there needs to be some clouds for the human eyes to see this. The science behind this is the same, with short-wavelength blue and violet light is scattered by molecules in the atmosphere while longer-wavelength red, orange, and pink light pass through and hit the clouds. If the clouds aren’t present, there’s nothing for the coloured light to reflect off.
Steven Ackerman, a professor of meteorology at UW–Madison, explains in one of his papers, how scattering helps us to see the sky change its colours at sunrise and sunset. “Because the sun is low on the horizon, sunlight passes through more air at sunset and sunrise than during the day, when the sun is higher in the sky. More atmosphere means more molecules to scatter the violet and blue light away from your eyes. If the path is long enough, all of the blue and violet light scatters out of your line of sight. The other colours continue on their way to your eyes. This is why sunsets are often yellow, orange, and red,” he writes.
And as we already know, red has the longest wavelength of any visible light, the sun appears to be red when it’s on the horizon, where its extremely long path through the atmosphere blocks all other colours.
Violet has the shortest wavelength, at around 380 nanometres while blue is between 450 to 495 nanometres. If short-wavelength blue light gets scattered then the sky should be violet and not blue. That’s because we can already see that violet light has the shortest wavelength of all. Well, the sky is actually violet, it is just that the human eye doesn’t perceive violet too well and is more sensitive to blue, hence the sky appears blue.
Other planets do not exactly have an atmosphere like the Earth’s, and thus their skies would look different. For instance, the atmosphere on Mars is much thinner than the Earth’s – less than one per cent. The low density of air molecules in their atmosphere means that the Rayleigh scattering that causes our skies to be blue here, has a very small effect on Mars. We might expect it to have a very faint blue coloured sky, but due to the haze of dust that remains suspended in the air, the daytime sky on Mars appears more yellowish.
Like the Moon’s atmosphere is negligibly thin, essentially considered a vacuum, so its sky is always black, as in the case of Mercury. Being closer to the Sun, Venus receives about 1.9 times more sunlight than our planet, but due to the thick atmosphere, only about 20 percent of the light reaches the surface. Colour images taken by the Soviet Venera, a series of space probes developed by the Soviet Union between 1961 and 1984, suggest that the sky on Venus is orange.
Have you ever observed the sky change its colour? How does it look to you? Tell us in the comments below.
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