Why skies are blue

The three different types of colour receptors in the retina of the human eye respond most strongly to red, green and blue wavelengths, giving us our colour vision. The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in He discovered that when light passes through a clear fluid holding small particles in suspension, the shorter blue wavelengths are scattered more strongly than the red. This can be demonstrated by shining a beam of white light through a tank of water with a little milk or soap mixed in.

From the side, the beam can be seen by the blue light it scatters; but the light seen directly from the end is reddened after it has passed through the tank. The scattered light can also be shown to be polarised using a filter of polarised light, just as the sky appears a deeper blue through polaroid sun glasses. 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. 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 yellow. This is because the larger dust particles absorb the short wavelength blue light, and scatter the remaining colours to give a butterscotch hue over the Martian sky.

When the air is too thin for gas molecules to collide with each other, we call it an 'exosphere' instead. But what makes the sea blue — is it reflecting the blue of the sky? Water molecules are good at absorbing longer wavelengths of light, so when sunlight hits the water the reds and oranges get absorbed.

The shorter wavelength blue light is absorbed very little and much of it is reflected back to our eyes. This article has been written by an astronomer at the Royal Observatory, Greenwich. Written and illustrated by astronomical experts, Storm Dunlop and Wil Tirion, and approved by the astronomers of Royal Observatory Greenwich Buy Now.

Special Price. Sky-Watcher Skyhawk Telescope. The ideal telescope of choice for beginner to intermediate astronomers who are looking to expand their skygazing experiences What is light? Closer to the horizon, the sky fades to a lighter blue or white. The sunlight reaching us from low in the sky has passed through even more air than the sunlight reaching us from overhead. As the sunlight has passed through all this air, the air molecules have scattered and re scattered the blue light many times in many directions.

Also, the surface of Earth has reflected and scattered the light. All this scattering mixes the colors together again so we see more white and less blue. As the Sun gets lower in the sky, its light is passing through more of the atmosphere to reach you. Even more of the blue light is scattered, allowing the reds and yellows to pass straight through to your eyes. Sometimes the whole western sky seems to glow.

The sky appears red because small particles of dust, pollution, or other aerosols also scatter blue light, leaving more purely red and yellow light to go through the atmosphere.

For example, Mars has a very thin atmosphere made mostly of carbon dioxide and filled with fine dust particles. During the daytime, the Martian sky takes on an orange or reddish color. But as the Sun sets, the sky around the Sun begins to take on a blue-gray tone.

The top image shows the orange-colored Martian sky during the daytime and the bottom image shows the blue-tinted sky at sunset.