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Asked by anon-252450 on 30 Apr 2020.
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Joel Goldstein answered on 30 Apr 2020:
Light from the sun scatters off small particles (dust etc.) in the atmosphere. The amount of light scattered depends on its wavelength – it is much stronger for short wavelengths (blue end of the spectrum) than for long wavelengths (the red end of the spectrum).
So as the white (i.e. all visible wavelengths) light coming from the sun passes through the atmosphere, a relatively large amount of the short (blue) wavelength light is scattered and bounces around before reaching us from all sorts of different directions – that is why the sky looks blue.
Also, the long wavelength (red) light tends to come straight through the atmosphere without scattering, so the sun looks more red than it really is: this is particularly noticeable at sunrise and sunset, when the sun is lower in the sky and so the light has to travel through more atmosphere.
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Susan Cartwright answered on 30 Apr 2020:
The sky is blue because the Earth’s atmosphere scatters incoming sunlight. The shorter the wavelength, the more the light is scattered, and this is a big effect (the amount of scattering is proportional to one over the fourth power of the wavelength), so blue light is scatter a LOT more than other colours. It is scattered so much that the direction from which it enters your eye is basically random, so the whole sky appears bluem even though the blue light has all come from the Sun. This is also why the Sun appears yellow – its colour when viewed from space is closer to white, but the blue is being scattered all over the sky whereas the other colours are still coming mostly from the direction of the Sun, and this makes the Sun’s light more yellow. It’s also why the rising and setting Sun appears red: when the Sun is low in the sky the light has to pass through much more atmosphere, and this allows the green and yellow light to be scattered over the sky as well as the blue – so the only light left obviously coming from the direction of the Sun is red.
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anon answered on 30 Apr 2020:
Because blue light gets scattered by our atmosphere, like pinball, while other colours (= wavelengths) can pass more freely. It is called “Rayleigh scattering”.
This is also why the sunsets and sunrises are orange/red – sunlight has to go through a thicker layer of air when the Sun is on the horizon and the blue light is scattered away while the redder light passes through.
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Greg Wallace answered on 1 May 2020:
The previous answers are correct: the process of light scattering off small particles of dust, called Rayleigh scattering, is much stronger for shorter wavelengths. However, shouldn’t this make the sky purple? The rainbow goes from long wavelength red to short wavelength purple, not blue.
The answer is that our eyes are a whole lot less sensitive to purple light than blue light, so we percieve the sky as bluer than it actually is!
To elaborate a bit; our eyes detect colours through ‘cone’ cells in the retinas of our eyes (as opposed to the ‘rod’ cells which don’t detect colour but are more sensitive and so help us see in the dark). We have three types of cone cell which detect red, green and blue light. They don’t just detect one colour though. They respond most strongly to their particular colour but they also respond to nearby colours, just less strongly. For example your red cone will still detect orange light, and your green cone will respond to it a very small amount as well. When our brain sees two types of cones responding it knows the colour must be somewhere in the middle, which is why if you mix two colours it appears as a different colour; it is just your brain being tricked.
The spectrum that each cone can detect overlaps with the next, which ensures that we see all the colours from red to blue more or less as strongly as each other. As the blue cone is at the edge though there is nothing to support it. Purple light is just the tail of the blue cone’s detection range
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