Why is the sky blue?

*Note: Project Weather lessons should always have adult supervision.

Throughout the past few weeks, we’ve focused in on a lot of experiments involving cold weather. We’ve made snow, and frost, and learned a lot about the water cycle. This week, I want to explore more of warm weather experiments!

Many children often wonder, 'Why is the sky blue?' I wondered the same thing as a child. It’s not an easy question to answer always, especially when trying to explain to a kid. It took scientists a long time to figure out the answer, and it all has to do with a prism and the way light bends through it.

Photo: Nasa.gov

When white light, or in our case sunlight, shines through a prism, all different colors of the rainbow are created as shown above. Some light travels in short waves (blue). Other light travels in long waves. Blue light waves are shorter than red light waves.

Photo: Nasa.gov

In order to put this in perspective of our world and atmosphere, think about it this way: sunlight reaches Earth and the light is scattered in different directions by everything floating in the air. Blue light is scattered more than the other colors because it travels in those shorter and smaller waves (shown above). This is why we see a blue sky when you look up.

Closer to the ground, the sky fades to a lighter ‘sky’ blue or even white. The reason for the color variation is that the sunlight has gone through a lot more hoops to get to the ground. As the rays moved through more air they have broken apart and scattered and then scattered again.

Since the surface of Earth has reflected and scattered the light, all this movement mixes the colors together so we see whiter coloring.

In this experiment we will try to recreate what happens outside by using:

  • A flashlight
  • An empty 2-liter bottle
  • Milk
  • Water

Why Is the Sky Blue?

You’ll start by filling up your pop bottle ¾ of the way full with water. From there, find a spot where you can put a flashlight on the side of the bottle so it shines through.

Why Is the Sky Blue?

Next, you will need to add a teaspoon of milk.

Why Is the Sky Blue?

Put the cap on the 2-liter and shake the milk and water mixture together. At this point set the bottle next to the flashlight again and look for scattering blue light.

Why is the sky blue?

[Physics FAQ] – [Copyright]

Original by Philip Gibbs May 1997.

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.

Why Is the Sky Blue?

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.

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Why Is the Sky Blue?

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.  But 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.

Why Is the Sky Blue?

Sunsets

Why Is the Sky Blue?

We see a blue sky, because of the way the atmosphere interacts with sunlight.

White light, including sunlight, is made up of many different colors of light, each with its own corresponding wavelength.

Several different things can happen when this light encounters matter.

For instance, if sunlight passes through a transparent material, such as water, those light waves will refract, or bend, because light changes speed as it travels from one medium (air) to another (water). Prisms break up white light into its constituent colors, because different wavelengths of light refract at different angles — the colors travel at different speeds — as they pass through the prism. 

Alternatively, some objects, such as mirrors, reflect light in a single direction. Other objects can cause light to scatter in many directions.

Why Is the Sky Blue?(Image credit: Karl Tate, Infographic Artist)

The blueness of the sky is the result of a particular type of scattering called Rayleigh scattering, which refers to the selective scattering of light off of particles that are no bigger than one-tenth the wavelength of the light.

Importantly, Rayleigh scattering is heavily dependent on the wavelength of light, with lower wavelength light being scattered most.

In the lower atmosphere, tiny oxygen and nitrogen molecules scatter short-wavelength light, such blue and violet light, to a far greater degree than than long-wavelength light, such as red and yellow.

In fact, the scattering of 400-nanometer light (violet) is 9.4 times greater than the scattering of 700-nm light (red).

Though the atmospheric particles scatter violet more than blue (450-nm light), the sky appears blue, because our eyes are more sensitive to blue light and because some of the violet light is absorbed in the upper atmosphere.

During sunrise or sunset, the sun's light has to pass through more of the atmosphere to reach your eyes. Even more of the blue and violet light gets scattered, allowing the reds and yellows to shine through.

Why is the sky blue?

Category: Earth Science      Published: March 28, 2013

If you look in any popular science book on this topic, it will tell you that the sky is blue because of Rayleigh scattering in the atmosphere.

While Rayleigh scattering is a very important part of the answer, it is not the only part. If the only effect at work were Rayleigh scattering, then the sky would be violet, not blue.

In fact, there are four factors involved; all required to give the final answer of blue:

  • Rayleigh scattering in the atmosphere
  • The incident sunlight spectrum is a thermal distribution
  • Bulk attenuation by the atmosphere
  • The human eyes and brain mix and perceive colors non-linearly

Rayleigh scattering is what happens when light bounces off an object that is much smaller than its wavelength. Physical derivations show that for Rayleigh scattering, the higher frequency colors such as blue and violet are scattered much more strongly than low frequency colors such as red and orange.

Mathematically, the intensity of scattered light is proportional to f 4, where f is the frequency of the light. This means that a color that is twice the frequency of another color will be 16 times brighter than that other color after scattering if they both were equally bright to start out with.

In the atmosphere, the objects doing the scattering are mostly nitrogen molecules (N2) and oxygen molecules (O2). These molecules are much smaller than visible light (oxygen molecules are about 0.1 nanometers wide and visible light has a wavelength of about 500 nanometers), so the scattering in the atmosphere is Rayleigh scattering.

But if this were the end of the story, the sky would be violet and not blue because violet is the highest-frequency visible color. Let us look at the other effects.

The color spectrum of the sky if only Rayleigh scattering were involved. The sky would look violet. It's a good thing there is more going on than Rayleigh scattering, otherwise we would be fried by all that ultraviolet radiation. The y axis is the brightness of a certain color. Public Domain Image, source: Christopher S. Baird.

Most people think that the sunlight traveling through space before it hits our atmosphere is a perfectly white color. Perfect white is an equal mix of all colors. In reality, the sunlight hitting the atmosphere does not have an equal distribution of colors, but has a thermal (black-body) distribution.

The sun is a big ball of incandescent gas very similar to a candle flame. The light that our sun sends out into to space is created by its heat, and therefore it has a thermal distribution of colors. The exact nature of a thermal distribution depends on the temperature of the glowing object (i.e. red-hot toaster elements vs. white-hot coals).

The sun's surface temperature is at about 5800 Kelvin, which gives a thermal distribution that peaks in the infrared (in frequency space). The sunlight that hits the atmosphere is therefore not an equal mix of all colors, but is a mix of all colors with red-orange dominating, and with more blue than violet (at least in frequency space).

This helps explain why the sky is blue and not violet. But it turns out to still not be enough.

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The color spectrum of the sky considering Rayleigh scattering + thermal incident sunlight. Public Domain Image, source: Christopher S. Baird.

Bulk attenuation means that as sunlight travels through the thick atmosphere, it becomes progressively weaker because it is being partially scattered all along the way. The rate at which it becomes weaker is faster for higher-frequency colors.

In other words, because blue and violet scatter the strongest, they are also removed the quickest from the forward traveling beam as it travels down through atmosphere. Bulk attenuation is what makes sunsets red and orange. At sunset, the sunlight is approaching an observer at a low angle, so it has to travel through much more atmosphere.

It travels through so much air in fact, that by the time the light reaches the layer of air closest to the grounded observer, all of the green, blue and violet colors have long since been scattered out by higher layers of atmosphere, leaving just red and orange.

While it is still true that the air in the layer of sky closest to the observer scatters blue and violet light the strongest, there is no blue or violet light left in the light beam to scatter at sunset because of bulk attenuation. Even during the day when the sun is high in the sky, bulk attenuation has an effect.

The sunlight in the forward beam near the bottom of the atmosphere has less violet than blue as compared to the beam when it was in the top of the atmosphere because the violet color scatters more quickly our of the forward beam. While this helps explain why the sky is blue and not violet, it is not enough.

The color spectrum of the sky considering Rayleigh scattering + thermal incident sunlight + bulk attenuation around noon near the equator. At sunset, the bulk attenuation effect shifts this entire curve to red colors. Public Domain Image, source: Christopher S. Baird.

Finally, human eyes perceive color in a non-linear fashion. For instance, if a blue spot on the wall sits next to a violet spot on the wall, and a laboratory tool measures them both to be equal brightness, our eyes will perceive the blue spot to be brighter.

In a loose sense, our eyes are more sensitive to blue light than violet light. But the situation is more complex than this statement makes it seem. Our eyes have three color receptors: red, green, and blue, but these receptors overlap quite a bit.

Our brains then mix the three signals non-linearly to give us the experience of color. In a simplified view, equal parts of red, green, and blue are perceived as white, whereas a little red plus a little green plus a lot of blue-violet is perceived as whitish-blue.

Our brains mix together the spectrum shown above to give us the sky blue shown below. All of the effects mentioned above are needed to explain why humans see the sky as blue.

Our eyes and brain combine the spectrum of colors in the previous image to give us this color. Public Domain Image, source: Christopher S. Baird.

Topics: Rayleigh scattering, blue, blue sky, bulk attenuation, color, light, scattering, sky, solar spectrum, vision

Why is the sky blue?

Anthony D. Del Genio of the NASA Goddard Institute for Space Studies and Columbia University explains.

To understand why the sky is blue, we need to consider the nature of sunlight and how it interacts with the gas molecules that make up our atmosphere.

Sunlight, which appears white to the human eye, is a mixture of all the colors of the rainbow.

For many purposes, sunlight can be thought of as an electromagnetic wave that causes the charged particles (electrons and protons) inside air molecules to oscillate up and down as the sunlight passes through the atmosphere.

When this happens, the oscillating charges produce electromagnetic radiation at the same frequency as the incoming sunlight, but spread over all different directions. This redirecting of incoming sunlight by air molecules is called scattering.

The blue component of the spectrum of visible light has shorter wavelengths and higher frequencies than the red component. Thus, as sunlight of all colors passes through air, the blue part causes charged particles to oscillate faster than does the red part.

The faster the oscillation, the more scattered light is produced, so blue is scattered more strongly than red. For particles such as air molecules that are much smaller than the wavelengths of visible light the difference is dramatic.

The acceleration of the charged particles is proportional to the square of the frequency, and the intensity of scattered light is proportional to the square of this acceleration. Scattered light intensity is therefore proportional to the fourth power of frequency.

The result is that blue light is scattered into other directions almost 10 times as efficiently as red light.

When we look at an arbitrary point in the sky, away from the sun, we see only the light that was redirected by the atmosphere into our line of sight. Because that occurs much more often for blue light than for red, the sky appears blue.

Violet light is actually scattered even a bit more strongly than blue.

More of the sunlight entering the atmosphere is blue than violet, however, and our eyes are somewhat more sensitive to blue light than to violet light, so the sky appears blue.

When we view the setting sun on the horizon, the opposite occurs. We see only the light that has not been scattered into other directions. The red wavelengths of sunlight that pass through the atmosphere without being scattered much reach our eyes, while the strongly scattered blue light does not.

The longer distance that the sunlight travels through the atmosphere when it is on the horizon amplifies the effect–there are more opportunities for blue light to be scattered than when the sun is overhead. Thus, the setting sun appears reddish.

In a polluted sky, small aerosol particles of sulfate, organic carbon, or mineral dust further amplify the scattering of blue light, making sunsets in polluted conditions sometimes spectacular.

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Clouds, on the other hand, are made of water droplets that are much larger than the wavelengths of visible light. The way they scatter sunlight is determined by how the light is refracted and internally reflected by, and diffracted around, the cloud droplets.

For these particles the difference between the scattering of blue and red light is not nearly so large as it is for gas molecules.

Hence, our eyes receive substantial scattered light at all visible wavelengths, causing clouds to appear more white than blue, especially when viewed against a blue sky background.

Since scattering by the atmosphere causes the sky to be blue, a planet with no atmosphere cannot have a bright sky. For example, photographs taken by the Apollo astronauts on the moon show them and the moon's surface bathed in sunlight, but a completely dark sky in all directions away from the sun.

Why is the sky blue?

To understand why the sky is blue, we first need to understand a little bit about light. Although light from the Sun looks white, it is really made up of a spectrum of many different colours, as we can see when they are spread out in a rainbow.

We can think of light as being a wave of energy, and different colours all have a different wavelength. At one end of the spectrum is red light which has the longest wavelength and at the other is blue and violet lights which have a much shorter wavelength.

When the Sun's light reaches the Earth's atmosphere it is scattered, or deflected, by the tiny molecules of gas (mostly nitrogen and oxygen) in the air. Because these molecules are much smaller than the wavelength of visible light, the amount of scattering depends on the wavelength. This effect is called Rayleigh scattering, named after Lord Rayleigh who first discovered it.

Shorter wavelengths (violet and blue) are scattered the most strongly, so more of the blue light is scattered towards our eyes than the other colours.

You might wonder why the sky doesn't actually look purple since the violet light is scattered even more strongly than blue.

This is because there isn't as much violet in sunlight to start with, and our eyes are much more sensitive to blue.

The blue light that gives the sky its colour, is sufficiently bright to make all the stars that we see at night disappear since the light they emit is much dimmer.

Why does the blue fade towards the horizon?

You might also notice that the sky tends to be most vibrant overhead and fades to pale as it reaches the horizon.

This is because the light from the horizon has had further to travel through the air and so has been scattered and rescattered. The Earth's surface also plays a role in scattering and reflecting this light.

As a result of this increased amount of scattering, the dominance of blue light is decreased and so we see an increased amount of white light.

Why is the sky blue?

  • As white light passes through our atmosphere, tiny air molecules cause it to ‘scatter’.
  • The scattering caused by these tiny air molecules (known as Rayleigh scattering) increases as the wavelength of light decreases.
  • Violet and blue light have the shortest wavelengths and red light has the longest.
  • Therefore, blue light is scattered more than red light and the sky appears blue during the day.
  • When the Sun is low in the sky during sunrise and sunset the light has to travel further through the Earth’s atmosphere.
  • We don’t see the blue light because it gets scattered away, but the red light isn’t scattered very much so the sky appears red.

The Sun gives out or emits all the colours of visible light which we see as being approximately white. As demonstrated by Sir Isaac Newton with a triangular prism, when white light passes through the prism it separates out into the colours of the rainbow.

This experiment demonstrates that white light is composed of all the colours of visible light in roughly the same amounts. These different colours have different wavelengths and this affects how they interact with different substances. Violet and blue light have the shortest wavelengths and red light has the longest.

Check out our video ‘What is light’ to learn about how light is more than what we see with just our eyes.

Scattering of light

The Earth’s atmosphere is composed of lots of different air molecules. Sunlight can be redirected by the air molecules and this is known as scattering.

The size of these molecules is much smaller than the wavelengths of visible light and so the type of scattering that occurs is known as Rayleigh scattering named after Lord Rayleigh (John William Strutt) who discovered it.

This type of scattering increases as the wavelength of light decreases so blue light is scattered more than red light by the tiny air molecules in our atmosphere.

The sky during the day

At noon, when the Sun is overhead it appears white. This is because the light travels a shorter distance through the atmosphere to get to us; it’s scattered very little, even the blue light.

During the day the sky looks blue because it’s the blue light that gets scattered the most.

It’s redirected into many different directions all over the sky, whereas the other wavelengths aren’t scattered as much.

In reality, violet light has a shorter wavelength compared to blue light and therefore it’s scattered more – so why isn’t the sky violet?

It’s because our eyes are actually more sensitive to detecting blue light and more of the sunlight coming into the Earth’s atmosphere is blue rather than violet.

Why does the sky look red during sunrise and sunset?

During sunrise or sunset, the sky appears to change colour.

When the Sun is low in the sky, the light has to travel a longer distance through the Earth’s atmosphere so we don’t see the blue light because it gets scattered away.

Instead we see the red and orange light that travels towards us since this light hasn’t been scattered very much. Hence the Sun and skies look redder at dawn and dusk.

Skies on other planets

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