Why is the sky blue?

Have you ever wondered why our sky is blue? The molecules in our atmosphere are made up of about 78% nitrogen, 21% oxygen, .7% argon, and a small amount of trace gases. White light from our sun is made up of all the colors in the rainbow from red to violet. When the light hits our atmosphere the molecules scatter blue light more than red or green etc. so we see a blue sky! Let's do an experiment to see the effect of scattering, and some other properties of light. Get as many of the following materials as possible: A 1 liter beaker or 1 qt glass jar (a 12 oz glass will do), some milk, a laser pointer or other low power laser, a prism or diffraction grating and some lenses, and a lab notebook to record your observations. Fill the beaker with cold tap water and shine your laser through it. Mix 2-3 drops of milk into the water. Take the mixture into a dimly lit, or dark, room. Shine the laser through the side of the beaker and look through the top of the beaker, jar, or glass. What do you see? The mixture you made is called a colloidal suspension. Now shine the laser up through the side of the beaker toward the center of surface of the suspension.
[Physics FAQ] - [Copyright]
Original by Philip Gibbs May 1997.

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 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.

A multicoloured sunset over the Firth of Forth in Scotland.
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.

A blue haze over the mountains of Les Vosges in France.
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.
Why is the Mars sky red?
Images sent back from the Viking Mars landers in 1977 and from Pathfinder in 1997 showed a red sky seen from the Martian surface. This was due to red iron-rich dusts thrown up in the dust storms occurring from time to time on Mars. The colour of the Mars sky will change according to weather conditions. It should be blue when there have been no recent storms, but it will be darker than the earth's daytime sky because of Mars' thinner atmosphere. http://math.ucr.edu/home/baez/physics/General/BlueSky/blue_sky.html Blue SkyThe blue color of the sky is caused by the scattering of sunlight off the molecules of the atmosphere. This scattering, called Rayleigh scattering, is more effective at short wavelengths (the blue end of the visible spectrum). Therefore the light scattered down to the earth at a large angle with respect to the direction of the sun's light is predominantly in the blue end of the spectrum. | Note that the blue of the sky is more saturated when you look further from the sun. The almost white scattering near the sun can be attributed to Mie scattering, which is not very wavelength dependent. | Measurement of the progression of saturation and brightness | Clouds in contrast to the blue sky appear white to achromatic gray. | |
The water droplets that make up the cloud are much larger than the molecules of the air and the scattering from them is almost independent of wavelength in the visible range. | Index

Scattering concepts

Atmospheric optics concepts | HyperPhysics***** Light and Vision | R Nave | | Go Back |

Rayleigh ScatteringRayleigh scattering refers to the scattering of light off of the molecules of the air, and can be extended to scattering from particles up to about a tenth of the wavelength of the light. It is Rayleigh scattering off the molecules of the air which gives us the blue sky. Lord Rayleigh calculated the scattered intensity from dipole scatterers much smaller than the wavelength to be: Rayleigh scattering can be considered to be elastic scattering since the photon energies of the scattered photons is not changed. Scattering in which the scattered photons have either a higher or lower photon energy is called Raman scattering. Usually this kind of scattering involves exciting some vibrational mode of the molecules, giving a lower scattered photon energy, or scattering off an excited vibrational state of a molecule which adds its vibrational energy to the incident photon. Compare with Mie scattering | | Index

Scattering concepts

Atmospheric optics concepts | HyperPhysics***** Light and Vision | R Nave | | Go Back |

Mie ScatteringThe scattering from molecules and very tiny particles (< 1 /10 wavelength) is predominantly Rayleigh scattering. For particle sizes larger than a wavelength, Mie scattering predominates. This scattering produces a pattern like an antenna lobe, with a sharper and more intense forward lobe for larger particles.Mie scattering is not strongly wavelength dependent and produces the almost white glare around the sun when a lot of particulate material is present in the air. It also gives us the the white light from mist and fog. Greenler in his "Rainbows, Haloes and Glories" has some excellent color plates demonstrating Mie scattering and its dramatic absence in the particle-free air of the polar regions. Compare with Rayleigh scattering | | Index

Scattering concepts

Atmospheric optics concepts

Meyer-Arendt
2nd Ed., Sec 3.3

Williamson & Cummins
Sec 14.4 | HyperPhysics***** Light and Vision | R Nave | | Go Back |

Rayleigh and Mie Scattering Mie Scattering | Rayleigh Scattering | Blue sky | Rayleigh and Mie scattering and sky color. | Possible Mie scattering from fogged eyeglasses | | Index

Scattering concepts

Atmospheric optics concepts | HyperPhysics***** Light and Vision | R Nave | | Go Back |

Sky Saturation and BrightnessAs a qualitative examination of sky brightness and the saturation of the blue sky color, measurements of the color of the sky photograph were made from a computer monitor using Adobe Illustrator's color tools. None of the data should be taken as quantitatively reliable since the original photo had been transformed several times, and the measurements were taken from a non-calibrated computer monitor. Nevertheless, it might be useful as an example of the progressions of sky color.A series of points on the sky image were chosen starting from the left, indicated by the white dots superimposed on the image above. It is clear to the eye that the progression leads to a brighter sky and to a blue color which is less saturated, or more pastel. Measurements of the color and brightness were made at each point based on amounts of red, green and blue present. In the graph at upper left, the blue brightness was normalized to 1 and the red and green expressed as a fraction of the blue. One result was that the green was significantly brighter than the red. This is consistent with Rayleigh scattering which emphasizes the shorter wavelengths. Another result was that the red and green increased as a fraction of the blue, indicating that the color was becoming less saturated. This can be interpreted as blue mixed with an increasing fraction of white light, which is consistent with the light being a combination of Rayleigh and Mie scattering. As you approach the sun's direction, the Mie scattering accounts for a larger fraction of the total light, and the Mie scattered light is essentially white. The graph of overall brightness above is just the sum of all three colors, with a maximum of 1 being white on the monitor. The increasing brightness along the path of the data is again consistent with a combination of Rayleigh and Mie scattering. The Mie scattering has a strong forward lobe and increases as you approach the sun's direction. Mie Scattering | Rayleigh Scattering | Blue sky | | Index

Scattering concepts

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http://hyperphysics.phy-astr.gsu.edu/hbase/atmos/blusky.html
Why Is the Sky Blue?
Rayleigh Scattering From Atmospheric Particles
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* Oct 7, 2007 * Paul A. Heckert
Small particles in the atmosphere scattering light cause the blue sky.
Blue Sky
Why is the sky blue? On a crystal clear day the sky is a lovely shade of Carolina blue. Many people wonder what causes this effect, but first let's consider a common myth about what does not cause the blue sky.
Light Reflected off the Ocean Does Not Cause the Blue Sky
There is a common myth that light reflects off the oceans to cause the blue sky. To understand why this can not be correct, think about the implications.
If the blue sky were in some way caused by oceans, then the sky would be a much richer shade of blue in coastal areas than in the middle of continents. For example on the North American continent California or any of the Atlantic coastal states would have bluer skies than Nebraska or Kansas, which are at the geographical center of North America. This author has personally verified that the sky can be just as blue in Nebraska and Kansas as in a coastal area. The oceans do not in any way cause blue skies. Particles in Earth's upper atmosphere scattering light cause the blue sky.
Rayleigh Scattering
When light bounces, or reflects, off small particles, it is called scattering. Rayleigh scattering is the type of scattering that occurs when the particles are smaller than the wavelength of the light being scattered. In 1871, Lord Rayleigh worked out the mathematical law that describes how Rayleigh scattering works.
Rayleigh found that shorter wavelength light is scattered much more efficiently than light at longer wavelengths. Mathematically, the amount of scattering is related to the fourth power of the wavelength. So blue light with a wavelength of about 400 nanometers is scattered about 10 times more efficiently than red light with a wavelength of about 700 nanometers.
Earth's atmosphere contains lots of oxygen and nitrogen molecules which are smaller than the wavelength of light. It also contains lots of dust particles. They are however usually larger than the wavelength of light so the Rayleigh scattering law does not apply to the dust particles. It is the molecules that cause the blue sky.
Read on * Why Are Sunsets and Sunrises Red? * Why the Sky on Mars Is Pink * Blue Skies, White Clouds, Red Sunsets
Why Is the Sky Blue?
White light coming from the Sun contains all colors of light from red to blue. The molecules in Earth's atmosphere do not scatter much of the red light, but they do scatter a significant amount of blue light.
The blue light might be scattered once or multiple times by different particles. A photon of blue light might bounce off one particle and be scattered in a random direction. It might then be scattered a second or even third time before coming down to Earth's surface.
When we look up to the sky, we see blue light that appears to come from all parts of the sky. This light is the blue light from the sun that has been scattered from oxygen and nitrogen molecules in the atmosphere. The process is random, so blue light appears to originate from the entire sky.
Knowing why the sky is blue can increase your enjoyment of a nice day. Remember the sky is Carolina blue not because God is a tarheel but because of Rayleigh scattering.
Further Reading
Why Are Sunsets and Sunrises Red
Knight, R., Physics for Scientists and Engineers, Pearson, 2004. Copyright Paul A. Heckert. Contact the author to obtain permission for republication.

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Why Are Sunsets and Sunrises Red?
Rayleigh Scattering From Atmospheric Particles
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* Oct 7, 2007 * Paul A. Heckert
Small particles in the atmosphere scattering light cause both sunsets and sunrises to be red.
Red Sunsets and Sunrises
We have all, at some time or another, enjoyed the beauty of a spectacular red sunrise or sunset. While watching nature's show many people will wonder what causes the sun to be so red when it is just above the horizon. Why are sunrises and sunsets red?
Particles in Earth's atmosphere cause this effect.
Rayleigh Scattering
When light bounces, or reflects, off small particles, it is called scattering. Rayleigh scattering is the type of scattering that occurs when the particles are smaller than the wavelength of the light being scattered. In 1871, Lord Rayleigh worked out the mathematical law that describes how Rayleigh scattering works. Rayleigh found that shorter wavelength light is scattered much more efficiently than light at longer wavelengths. Mathematically, the amount of scattering is related to the fourth power of the wavelength. So blue light with a wavelength of about 400 nanometers is scattered about 10 times more efficiently than red light with a wavelength of about 700 nanometers.
Earth's atmosphere contains lots of oxygen and nitrogen molecules which are smaller than the wavelength of light. It also contains lots of dust particles. They are however usually larger than the wavelength of light so the Rayleigh scattering law does not apply to the dust particles.
Why Are Sunsets and Sunrises Red?
White light coming from the Sun contains all colors of light from red to blue. The molecules in Earth's atmosphere do not scatter much of the red light, but they do scatter a significant amount of blue light. This effect causes the blue sky.
These molecules scattering light also cause the Sun to appear redder than it really is. When the Sun is high in the sky the amount of reddening is small. However the Sun will still appear redder from the ground than from space because the atmosphere scatters some of the blue light.
Read on * Why the Sky on Mars Is Pink * Planck's Law * Blue Skies, White Clouds, Red Sunsets
When the Sun is low in the sky, this effect from Rayleigh scattering is much greater. Much more of the blue light coming from the Sun is scattered away from the direct path towards our eyes. Hence the Sun will appear very red when it is low in the sky.
The same thing happens to the Moon. It often appears very red when it is low in the sky for the same reason.
Much of the beauty of red sunsets or sunrises comes not from the red Sun but from the red clouds just above the western or eastern horizon. Red sunlight reflecting off these clouds gives them the same rich red color the Sun has near the horizon. So in a spectacular sunset or sunrise the entire horizon is red.
The effect can increase when the Sun sets or rises over a large city. Pollution particles increase the effect by scattering or absorbing more blue light than red light.
Enjoy your next spectacular red sunrise or sunset and remember that Rayleigh scattering off molecules in Earth's atmosphere causes the effect.
Further Reading
Knight, R., Physics for Scientists and Engineers, Pearson, 2004. Copyright Paul A. Heckert. Contact the author to obtain permission for republica

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Blue Skies, White Clouds, Red Sunsets
Some Effects of Colloid Sized Particles
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* Sep 2, 2008 * Anthony Toole

Cloudscape - Anthony Toole
Salty water is a clear solution. Muddy water is opaque, because its coarser particles are a thousand times bigger than those of salt.
A third kind of mixture, a colloidal system, has particles with sizes in the intermediate range. These systems transmit light, and sometimes appear transparent, but are often translucent if the suspended particles are larger.
Examples of Colloids
The word, ‘colloid’, comes from the Greek word for ‘glue’. Indeed glues and gums, when suspended in a solvent, are colloidal in nature.
Paints and jellies consist of solid colloidal particles suspended in liquid. An emulsion, like milk, has liquid particles suspended in a liquid. When colloidal sized gas bubbles are dispersed through a liquid, a foam is produced, like whipped cream. Colloidal gas bubbles in a solid, on the other hand, produce a meringue or pumice. Pearls and coloured glasses contain solid particles in a solid medium. In fogs, mists and clouds, liquid particles are suspended in a gas, while smoke is an example of an aerosol, in which the colloid is a solid. Brownian Movement
Sometimes, colloidal particles can be detected under a microscope, because of their ability to scatter light sideways. If smoke is observed through a microscope, while illuminated by a bright light, tiny pinpoints can be seen jumping about continuously. This ceaseless motion is known as Brownian movement, and is caused by the smoke particles being bombarded by tiny, invisible, but rapidly moving molecules of air.
Clouds are made of colloidal water droplets and ice crystals. If the temperature falls, water vapour from the surrounding air condenses onto these, increasing their masses until they can no longer remain suspended, and fall as rain.
Colloids and Electricity
Sometimes, colloid particles gather electric charges onto their surfaces. Repulsion between these charges keeps the particles separated. If the charges can be neutralised, the particles coagulate and drop out of suspension.
Read on * Why is the Sky Blue? * Why Are Sunsets and Sunrises Red? * Why Is the Sky Blue?
Colloidal clay in river water is precipitated by the charged ions in salt, as the river enters the sea, so that estuaries are usually surrounded by extensive mud flats. Similarly, a salt like aluminium sulphate is added to domestic water to precipitate colloidal materials and so clarify the water. Adding salt to a wound helps blood to clot.
Scattering of Light
Light scattering is an important property of colloidal systems. Light passes unhindered through a solution, and is blocked completely by a coarse suspension. If the particles in suspension are colloidal, then the path of a light beam can be seen when viewed from the side, a phenomenon known as the Tyndall effect. The appearance of sunbeams is an example of this, in which light is scattered by particles of mist or dust suspended in the atmosphere.
White light consists of radiation of different wavelengths. The shorter wavelengths give rise to blue light and the longer to red light. Colloidal particles scatter blue light more effectively than red. Smoke from a burning object thus appears blue. When the particles are in the larger colloidal range, like water droplets in a cloud, all wavelengths of light are scattered, so the cloud appears white.
Suspended in air are particles of dust, ice crystals and water droplets, in the lower range of colloidal size. These allow the passage of light of longer wavelengths, but scatter blue light very effectively.
As sunlight passes through the atmosphere, this blue light is scattered towards the ground, causing the sky to appear blue. As the sun sinks, the sideways scattering of blue light leaves the red to continue, giving us the fiery spectacle of a glorious sunset. Copyright Anthony Toole. Contact the author to obtain permission for republication.

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Inquirying into Why Sun Light is Clear and the Sky is Blue
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* Jun 20, 2008 * David R. Wetzel

Blue Sky - Melissa Nykorchuk
Understanding how the different wavelengths of the colors in sunlight causes the sky to appear blue as it enters our atmosphere, is an important science concept.
A common question every parent and teacher has heard from children at some point; why is the sky blue? Learning the correct answer is important. Because once children hear an answer several times, right or wrong, this answer becomes embedded in their brain. Convincing them otherwise, if they learned the wrong answer, takes a lot of evidence to undo the wrong.
Common Answers to Why the Sky is Blue
Children come up with a variety of answers when asked why the sky is blue. Their answers are a direct result of what they hear from peers, parents, and movies or television. Here are a few common answers children give: * The sunlight reflects off the oceans * The sky is blue because it is the bottom of space * Because of all the water in the sky * The sky reflects off the top of clouds which contain water
Sunlight has Colors?
The sky appears blue because the molecules of air in the atmosphere scatter the blue portion of light as sunlight passes through the atmosphere. * This answer will illicit another response from children – but light is clear because you do not look blue!
It is true light is clear, because of all the colors which make up sunlight. At this point, children will want proof. This can be proved by using a prism to separate the clear sunlight into the colors of the rainbow – ROYGBIV.
You have probably already predicted the next question children will ask. This question is, why isn’t the sky red, yellow, orange, green, etc.? This is typical and expected from children who are now questioning the evidence being presented to them. They are conducting inquiry-based science. For more about inquiry-based science visit Understanding Science Inquiry.
Inquiring into ROYGBIV
Children need to understand the colors of the rainbow have different wave lengths. This will help them understand why only blue light is scattered by the atmosphere. One way to help them understand is to have them do the following:
Read on * Dispersion & Refraction in Optics * John Rayleigh : Inert Gas Argon * Why Are Sunsets and Sunrises Red? * On a sheet of paper write ROYGBIV vertically for the colors of the rainbow, starting at the top left and going down the left side of the paper. * Next have them draw one complete wave about 3 inches wide after the R for red. * Next have them draw the another complete wave a little shorter after the O for orange. * They continue drawing waves a little shorter width every time all the wave down to the V for Violet. * The final wave width for violet should be about 1/2 inch wide.
When you describe shorter and longer wavelengths they have a picture to look at, which they personally drew. This is important for inquiry-based science teaching and learning – personal experience.
Understanding Why the Sky is Blue
Now describe how as light passes through the atmosphere, most of the sunlight gets through. However some of the blue light is scattered by the air, because its wavelength is shorter than the ROYGBIV colors. * The next question is why nor Violet, because its wavelength is shorter than blue?
The answer to this is the human eye is strongly sensitive to red, green, and blue. This is where color vision for humans is derived. Humans cannot see violet very well; this is why the sky is not violet.
Connecting the prism and wavelengths is important to understanding why the sky is blue. Children need this evidence to support internalizing the new information they learned about sunlight and why the sky is blue. Copyright David R. Wetzel. Contact the author to obtain permission for republication.

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