Why is the sky blue

A clear cloudless day-time sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. When we look towards the sun at sunset, we see red and orange colours because the blue light has been scattered out and away from the line of sight. The white light from the sun is a mixture of all colours of the rainbow. This was demonstrated by Isaac Newton, who used a prism to separate the different colours and so form a spectrum.

The colours of light are distinguished by their different wavelengths. The visible part of the spectrum ranges from red light with a wavelength of about nm, to violet with a wavelength of about nm, with orange, yellow, green, blue and indigo between. 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. Blue is scattered more than other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

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. Put these three things together, and a blue sky is inevitable. Here's how it all comes together. Light of many different wavelengths, not all of which are visible, are emitted by the Sun. Sunlight is made up of all the different colors of light The photosphere of our Sun is so hot, at nearly 6, K, that it emits a wide spectrum of light, from ultraviolet at the highest energies and into the visible, from violet all the way to red, and then deep into the infrared portion of the spectrum.

The highest energy light is also the shortest-wavelength and high-frequency light, while the lower energy light has longer-wavelengths and low-frequencies than the high-energy counterparts. When you see a prism split up sunlight into its individual components, the reason the light splits at all is because of the fact that redder light has a longer wavelength than the bluer light. Schematic animation of a continuous beam of light being dispersed by a prism.

If you had ultraviolet The fact that light of different wavelengths responds differently to interactions with matter proves extremely important and useful in our daily lives. The large holes in your microwave allow short-wavelength visible light in-and-out, but keep longer-wavelength microwave light in, reflecting it. The thin coatings on your sunglasses reflect ultraviolet, violet, and blue light, but allow the longer-wavelength greens, yellows, oranges, and reds to pass through.

And the tiny, invisible particles that make up our atmosphere — molecules like nitrogen, oxygen, water, carbon dioxide, as well as argon atoms — all scatter light of all wavelengths, but scatter the shorter-wavelength light much more efficiently.

When the Sun is high overhead, the sky towards the zenith is a much darker blue, while the sky This is due to the larger amount of atmosphere, and the larger amount of scattered light, that is visible at low angles on the sky. Because these molecules are all much smaller than the wavelength of light itself, the shorter the light's wavelength is, the better it scatters. In fact, quantitatively, it obeys a law known as Rayleigh scattering , which teaches us that the violet light at the short-wavelength limit of human vision scatters more than nine times more frequently than the red light at the long-wavelength limit.

Some opalescent materials, like the one shown here, have similar Rayleigh scattering properties to When the Sun is high in the sky, this is why the entire sky is blue.

It appears a brighter blue the farther away from the Sun you look, because there's more atmosphere to see and therefore more blue light in those directions. In any direction you look, you can see the scattered light coming from the sunlight striking the entirety of the atmosphere between your eyes and where outer space begins.

This has a few interesting consequences for the color of the sky, depending on where the Sun is and where you're looking. From very high altitudes in the pre-sunrise or post-sunset skies, a spectrum of colors can be seen, If the Sun is below the horizon, the light all has to travel through large amounts of atmosphere.