Why is the Sky Blue?

Suddenly a thought strike in my mind when I am listening the song - "Aasma Hai Neela Kyun, Paani Geela Geela Kyun", that why the color of the sky is blue? So I got curious about it and I ended up my curiosity by writing this.

From our childhood we hear in many stories and rhymes that the sky color is blue but we don't know the reason behind it?

There are many misconceptions and incorrect answers — because it reflects the ocean; because oxygen is a blue-colored gas; because sunlight has a blue tint — while the right answer is often thoroughly overlooked. 

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 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.

The visible part of the spectrum ranges from red light with a wavelength of about 720 nm, to violet with a wavelength of about 380 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 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.

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.

While sunlight falls everywhere on the day side of Earth’s atmosphere, the redder wavelengths of light are only 11% as likely to scatter, and therefore make it to your eyes, as the violet light is.

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 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.

Tyndall Effect

The first steps towards correctly explaining the colour of the sky were taken by John Tyndall in 1859.  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.  

Dust or Molecules?

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 1911, 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.

Why not violet?

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.

What makes sunset red?

When the air is clear the sunset will appear yellow, because the light from the sun has passed a long distance through air and some of the blue light has been scattered away.  If the air is polluted with small particles, natural or otherwise, the sunset will be more red.  Sunsets over the sea may also be orange, due to salt particles in the air, which are effective Tyndall scatterers.  The sky around the sun is seen reddened, as well as the light coming directly from the sun.  This is because all light is scattered relatively well through small angles—but blue light is then more likely to be scattered twice or more over the greater distances, leaving the yellow, red and orange colours.

   

The End Notes

I hope that this information satisfy your curiosity about why the sky is blue. Hope you liked it. In your childhood if you asked this question to anybody what were their answers, share with us in comments.