The sky appears blue to the human eye as the short waves of blue light are scattered more than the other colours in the spectrum, making the blue light more visible. To understand why the sky is blue, we first need to understand a little bit about light.
Although light from the Sun looks white, it is really made up of a spectrum of many different colours, as we can see when they are spread out in a rainbow.
We can think of light as being a wave of energy, and different colours all have a different wavelength. At one end of the spectrum is red light which has the longest wavelength and at the other is blue and violet lights which have a much shorter wavelength. This scattering refers to the scattering of electromagnetic radiation of which light is a form by particles of a much smaller wavelength. These shorter wavelengths correspond to blue hues, hence why when we look at the sky, we see it as blue.
At sunset and sunrise, the angle at which sunlight enters the atmosphere is significantly changed, and most of the blue and green shorter wavelengths of light are scattered even before reaching the lower atmosphere, so we see more of the orange and red colours in the sky. The ocean is not blue because it reflects the sky, though I believed that up until a few years ago.
Water actually appears blue due to its absorption of red light. When light hits water, the water's molecules absorb some of the photons from the light.
This is most correctly called the Tyndall effect, but it is more commonly known to physicists as Rayleigh scattering—after Lord Rayleigh, who studied it in more detail a few years later. He showed that the amount of light scattered is inversely proportional to the fourth power of wavelength for sufficiently small particles. Tyndall and Rayleigh thought that the blue colour of the sky must be due to small particles of dust and droplets of water vapour in the atmosphere.
Even today, people sometimes incorrectly say that this is the case. Later scientists realised that if this were true, there would be more variation of sky colour with humidity or haze conditions than was actually observed, so they supposed correctly that the molecules of oxygen and nitrogen in the air are sufficient to account for the scattering. The case was finally settled by Einstein in , who calculated the detailed formula for the scattering of light from molecules; and this was found to be in agreement with experiment.
He was even able to use the calculation as a further verification of Avogadro's number when compared with observation. The molecules are able to scatter light because the electromagnetic field of the light waves induces electric dipole moments in the molecules. If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light.
Our eyes are also less sensitive to violet. That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works. We have three types of colour receptors, or cones, in our retina. They are called red, blue and green because they respond most strongly to light at those wavelengths.
As they are stimulated in different proportions, our visual system constructs the colours we see. We thus see the sky as a whitish-blue semi-transparent layer.
When standing on the surface of the earth and looking up at the daytime sky, you may be tempted to think that the sky is opaque. However, the daytime sky often looks like a uniform, featureless stretch of blue only because there are so few bright objects beyond the sky. The stars are so dim that their light is overpowered during the day by the sunlight scattered by the sky. The semi-transparent nature of the daytime sky is made obvious by the fact that you can see the moon during the day from the surface of the earth.
And as expected, the moon as seen from the surface of the earth during the day is tinted blue. You can see the moon just fine through the daytime sky, indicting that it is semi-transparent. The atmosphere between you and the moon just causes the moon to gain a blue tint during the day. For the exact same reason, an astronaut in space that is looking down at earth's surface can see mountains and deserts just fine, but he sees them as tinted blue.
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