Why is the sky blue?

On a clear sunny day, the sky above us looks bright blue. In the evening, the sunset puts on a brilliant show of reds, pinks and oranges. Why is the sky blue? What makes the sunset red?

THE ATMOSPHERE

The atmosphere is the mixture of gas molecules and other materials surrounding the earth. It is made mostly of the gases nitrogen (78%), and oxygen (21%). Argon gas and water (in the form of vapor, droplets and ice crystals) are the next most common things. There are also small amounts of other gases, plus many small solid particles, like dust, soot and ashes, pollen, and salt from the oceans.

The composition of the atmosphere varies, depending on your location, the weather, and many other things. There may be more water in the air after a rainstorm, or near the ocean. Volcanoes can put large amounts of dust particles high into the atmosphere. Pollution can add different gases or dust and soot.

The atmosphere is densest (thickest) at the bottom, near the Earth. It gradually thins out as you go higher and higher up. There is no sharp break between the atmosphere and space.

LIGHT WAVES

Light is a kind of energy that radiates, or travels, in waves. Many different kinds of energy travel in waves. For example, sound is a wave of vibrating air. Light is a wave of vibrating electric and magnetic fields. It is one small part of a larger range of vibrating electromagnetic fields. This range is called the electromagnetic spectrum.

Electromagnetic waves travel through space at 299,792 km/sec (186,282 miles/sec). This is called the speed of light. 

The energy of the radiation depends on its wavelength and frequency. Wavelength is the distance between the tops (crests) of the waves. Frequency is the number of waves that pass by each second. The longer the wavelength of the light, the lower the frequency, and the less energy it contains.

COLORS OF LIGHT

Visible light is the part of the electromagnetic spectrum that our eyes can see. Light from the sun or a light bulb may look white, but it is actually a combination of many colors. We can see the different colors of the spectrum by splitting the light with a prism. The spectrum is also visible when you see a rainbow in the sky. 

The colors blend continuously into one another. At one end of the spectrum are the reds and oranges. These gradually shade into yellow, green, blue, indigo and violet. The colors have different wavelengths, frequencies, and energies. Violet has the shortest wavelength in the visible spectrum. That means it has the highest frequency and energy. Red has the longest wavelength, and lowest frequency and energy.

LIGHT IN THE AIR

Light travels through space in a straight line as long as nothing disturbs it. As light moves through the atmosphere, it continues to go straight until it bumps into a bit of dust or a gas molecule. Then what happens to the light depends on its wave length and the size of the thing it hits.

Dust particles and water droplets are much larger than the wavelength of visible light. When light hits these large particles, it gets reflected, or bounced off, in different directions. The different colors of light are all reflected by the particle in the same way. The reflected light appears white because it still contains all of the same colors.

Gas molecules are smaller than the wavelength of visible light. If light bumps into them, it acts differently. When light hits a gas molecule, some of it may get absorbed. After awhile, the molecule radiates (releases, or gives off) the light in a different direction. The color that is radiated is the same color that was absorbed. The different colors of light are affected differently. All of the colors can be absorbed. But the higher frequencies (blues) are absorbed more often than the lower frequencies (reds). This process is called Rayleigh scattering. (It is named after Lord John Rayleigh, an English physicist, who first described it in the 1870's.)

WHY IS THE SKY BLUE?

The blue color of the sky is due to Rayleigh scattering. As light moves through the atmosphere, most of the longer wavelengths pass straight through. Little of the red, orange and yellow light is affected by the air.

However, much of the shorter wavelength light is absorbed by the gas molecules. The absorbed blue light is then radiated in different directions. It gets scattered all around the sky. 

Whichever direction you look, some of this scattered blue light reaches you. Since you see the blue light from everywhere overhead, the sky looks blue. 

As you look closer to the horizon, the sky appears much paler in color. To reach you, the scattered blue light must pass through more air. Some of it gets scattered away again in other directions. Less blue light reaches your eyes. The color of the sky near the horizon appears paler or white. 

THE BLACK SKY AND WHITE SUN

On Earth, the sun appears yellow. If you were out in space, or on the moon, the sun would look white. In space, there is no atmosphere to scatter the sun's light. On Earth, some of the shorter wavelength light (the blues and violets) are removed from the direct rays of the sun by scattering. The remaining colors together appear yellow.

Also, out in space, the sky looks dark and black, instead of blue. This is because there is no atmosphere. There is no scattered light to reach your eyes. 

WHY IS THE SUNSET RED?

As the sun begins to set, the light must travel farther through the atmosphere before it gets to you. More of the light is reflected and scattered. As less reaches you directly, the sun appears less bright. The color of the sun itself appears to change, first to orange and then to red. This is because even more of the short wavelength blues and greens are now scattered. Only the longer wavelengths are left in the direct beam that reaches your eyes. 

The sky around the setting sun may take on many colors. The most spectacular shows occur when the air contains many small particles of dust or water. These particles reflect light in all directions. Then, as some of the light heads towards you, different amounts of the shorter wavelength colors are scattered out. You see the longer wavelengths, and the sky appears red, pink or orange

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 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.

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.  It follows that blue light is scattered more than red light by a factor of (700/400)⁴ ~= 10.

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.

Response curves for the three types of cone in the human eye

When we look up at the sky, the red cones respond to the small amount of scattered red light, but also less strongly to orange and yellow wavelengths.  The green cones respond to yellow and the more strongly scattered green and green-blue wavelengths.  The blue cones are stimulated by colours near blue wavelengths, which are very strongly scattered.  If there were no indigo and violet in the spectrum, the sky would appear blue with a slight green tinge.  However, the most strongly scattered indigo and violet wavelengths stimulate the red cones slightly as well as the blue, which is why these colours appear blue with an added red tinge.  The net effect is that the red and green cones are stimulated about equally by the light from the sky, while the blue is stimulated more strongly.  This combination accounts for the pale sky blue colour.  It may not be a coincidence that our vision is adjusted to see the sky as a pure hue.  We have evolved to fit in with our environment; and the ability to separate natural colours most clearly is probably a survival advantage.

Sunsets

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.

Blue Haze and Blue Moon

Clouds and dust haze appear white because they consist of particles larger than the wavelengths of light, which scatter all wavelengths equally (Mie scattering).  But sometimes there might be other particles in the air that are much smaller.  Some mountainous regions are famous for their blue haze.  Aerosols of terpenes from the vegetation react with ozone in the atmosphere to form small particles about 200 nm across, and these particles scatter the blue light.  A forest fire or volcanic eruption may occasionally fill the atmosphere with fine particles of 500—800 nm across, being the right size to scatter red light.  This gives the opposite to the usual Tyndall effect, and may cause the moon to have a blue tinge since the red light has been scattered out.  This is a very rare phenomenon, occurring literally once in a blue moon.

Opalescence

The Tyndall effect is responsible for some other blue coloration's in nature: such as blue eyes, the opalescence of some gem stones, and the colour in the blue jay's wing.  The colours can vary according to the size of the scattering particles.  When a fluid is near its critical temperature and pressure, tiny density fluctuations are responsible for a blue coloration known as critical opalescence.  People have also copied these natural effects by making ornamental glasses impregnated with particles, to give the glass a blue sheen.  But not all blue colouring in nature is caused by scattering.  Light under the sea is blue because water absorbs longer wavelength of light through distances over about 20 metres.  When viewed from the beach, the sea is also blue because it reflects the sky, of course.  Some birds and butterflies get their blue colorations by diffraction effects

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