why is the sky blue?

Answer 1

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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?

To answer these questions, we must learn about light, and the Earth's atmosphere.



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.



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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.



Blue Sky
Now you can explain why the sky is blue and the sunset is red





When sunlight travels through the atmosphere, blue light scatters more than the other colors, leaving a dominant yellow-orange hue to the transmitted light. The scattered light makes the sky blue; the transmitted light makes the sunset reddish orange.




A transparent plastic box, or a large beaker, jar, or aquarium

A flashlight or projector (either a slide or filmstrip projector)

Powdered milk

Polarizing filter (such as the lens from an old pair of polarized sunglasses)

Blank white card for image screen

Paper hole-punch

Optional: Unexposed (black) 35 mm slide or photographic film, or an index card cut to slide size



(15 minutes or less)

Fill the container with water. Place the light source so that the beam shines through the container. Add powdered milk a pinch at a time; stir until you can clearly see the beam shining through the liquid.






(15 minutes or more)



Look at the beam from the side of the tank and then from the end of the tank. You can also let the light project onto a white card, which you hold at the end of the tank. From the side, the beam looks bluish-white; from the end, it looks yellow-orange.

If you have added enough milk to the water, you will be able to see the color of the beam change from blue-white to yelloworange along the length of the beam.

If you want to look at a narrower beam of light, use a paper hole-punch to punch a hole in the unexposed black slide or in a piece of 35 mm film, or even in an index card cut to size. Place the slide, film, or index card in the projector. (Do not hold it in front of the lens.) Focus the projector to obtain a sharp beam.





The sun produces white light, which is made up of light of all colors: red, orange, yellow, green, blue, indigo, violet. Light is a wave, and each of these colors corresponds to a different frequency, and therefore wavelength, of light. The colors in the rainbow spectrum are arranged according to their frequency: violet, indigo, and blue light have a higher frequency than red, orange, and yellow light.

When the white light from the sun shines through the earth's atmosphere, it collides with gas molecules. These molecules scatter the light.



The shorter the wavelength of light, the more it is scattered by the atmosphere. Because it has a shorter wavelength, blue light is scattered ten times more than red light.

Blue light also has a frequency that is closer to the resonant frequency of atoms than that of red light. That is, if the electrons bound to air molecules are pushed, they will oscillate with a natural frequency that is even higher than the frequency of blue light. Blue light pushes on the electrons with a frequency that is closer to their natural resonant frequency than that of red light. This causes the blue light to be reradiated out in all directions, in a process called scattering. The red light that is not scattered continues on in its original direction. When you look up in the sky, the scattered blue light is the light that you see.

Why does the setting sun look reddish orange? When the sun is on the horizon, its light takes a longer path through the atmosphere to your eyes than when the sun is directly overhead. By the time the light of the setting sun reaches your eyes, most of the blue light has been scattered out. The light you finally see is reddish orange, the color of white light minus blue.

Violet light has an even shorter wavelength than blue light: It scatters even more than blue light does. So why isn't the sky violet? Because there is just not enough of it. The sun puts out much more blue light than violet light, so most of the scattered light in the sky is blue.




Scattering can polarize light. Place a polarizing filter between the projector and the tank. Turn the filter while one person views the transmitted beam from the top and another views it from the side. Notice that when the top person sees a bright beam, the side person will see a dim beam, and vice versa.

You can also hold the polarizing filter between your eyes and the tank and rotate the filter to make the beam look bright or dim. The filter and the scattering polarize the light. When the two polarizations are aligned, the beam will be bright; when they are at right angles, the beam will be dim.

Scattering polarizes light because light is a transverse wave. The direction of the transverse oscillation of the electric field is called the direction of polarization of light.



The beam of light from the slide projector contains photons of light that are polarized in all directions. horizontally, vertically, and all angles in between. Consider only the vertically polarized light passing through the tank. This light can scatter to the side and remain vertically polarized, but it cannot scatter upward! To retain the characteristic of a transverse wave after scattering, only the vertically polarized light can be scattered sideways, and only the horizontally polarized light can be scattered upward. This is shown in the drawing.



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It is easy to see that the sky is blue. Have you ever wondered why? A lot of other smart people have, too. And it took a long time to figure it out!

The light from the Sun looks white. But it is really made up of all the colors of the rainbow.



A prism is a specially shaped crystal. When white light shines through a prism, the light is separated into all its colors.

If you visited The Land of the Magic Windows, you learned that the light you see is just one tiny bit of all the kinds of light energy beaming around the Universe--and around you!

Like energy passing through the ocean, light energy travels in waves, too. Some light travels in short, "choppy" waves. Other light travels in long, lazy waves. Blue light waves are shorter than red light waves.



All light travels in a straight line unless something gets in the way to--


reflect it (like a mirror)

bend it (like a prism)

or scatter it (like molecules of the gases in the atmosphere)

Sunlight reaches Earth's atmosphere and is scattered in all directions by all the gases and particles in the air. Blue light is scattered in all directions by the tiny molecules of air in Earth's atmosphere. 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 rescattered 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.

What Makes a Red Sunset?

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 larger particles of dust, pollution, and water vapor in the atmosphere reflect and scatter more of the reds and yellows.

Why Does Scattering Matter?

How much of the Sun's light gets bounced around in Earth's atmosphere and how much gets reflected back into space? How much light gets soaked up by land and water, asphalt freeways and sunburned surfers? How much light do water and clouds reflect back into space? And why do we care?
Sunlight carries the energy that heats Earth and powers all life on Earth. Our climate is affected by how sunlight is scattered by forests, deserts, snow- and ice-covered surfaces, different types of clouds, smoke from forest fires, and other pollutants in the air.


MISR (for Multi-angle Imaging SpectroRadiometer) is one of five instruments onboard the Terra satellite. MISR has nine separate cameras that take pictures of Earth's atmosphere and surface from different angles as the Terra satellite passes over a region during its orbit.



MISR flies over a brush fire burning in Northern California.

Answer 2

Sky is blue only during day time and not at night. During the day, the white light from the sun is scattered by the air molecules ( mostly nitrogen and oxygen). Scattering of light is dependent on the wave length. White light consists of many colors and they are differently scattered. Blue is scattered more than red and green, thus giving the sky its blue look.

In mornings and in evenings, the part of the sky where sun is looks reddish (orange) because sunlight has to pass through a longer distance of atmosphere and the reddish portion of the light becomes more predominant (the blue gets scattered out).

Answer 3

Light is an electromagnetic wave. If you stand in one spot as a light wave passes by, there will be an oscillating electric field and an oscillating magnetic field, which are perpendicular to each other. If the light is in the range of frequencies that we can see, then the frequency of the vibration affects the color of the light. The color-vision receptors in our eyes, the cones, are of three types: "blue" receptors that respond to light over a broad range of high frequencies, "green" receptors that respond to medium frequencies, and "red" receptors that respond to low frequencies. The ranges of sensitivity of the receptors overlap considerably, but they have their maximum sensitivities at different frequencies. The perceived color depends (among other things) on the relative strengths of the signals from these receptors.
Molecules are usually electrically neutral, but they are made of charged objects: their atoms consist of negatively charged electrons and positively charged nuclei. If there is an electric field at the position of an atom, the nucleus will move a short distance in the direction of the field and the electrons will move the other way, and the atom will become a "dipole": the positive and negative charge will be centered around different places. A molecule made of such atoms will acquire its own electric field, something like the magnetic field of a bar magnet.
A dipole's electric field falls off more rapidly with distance than it would if the molecule had a net electric charge. This is because at large distances, the fields from the positive and the negative charge tend to cancel each other out, as the difference between their average positions becomes less important.
However, if the dipole is made to oscillate-- that is, if the positive and negative charge wiggle back and forth, out of phase with each other-- then the molecule can produce electromagnetic radiation of its own. This is how air molecules scatter light: the oscillating electric field of the incoming wave makes the molecules develop oscillating dipoles, which in turn give off radiation.
The radiation destructively interferes with the incoming wave in the forward direction. The original wave is lessened in intensity, and new waves move out in all other directions, so that overall energy is conserved (this requirement is sometimes called the "optical theorem"). The net effect is that light energy that was moving in a straight line from the sun ends up traveling in some other direction.
Since sunlight appears white but the sky is a robin's-egg blue, it must be that the scattered light excites our blue-sensing cones more, and our red-sensing cones less, than the original sunlight. The distribution of frequencies in the scattered light must be biased toward high frequencies.

Answer 4

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.

Answer 5

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.

Answer 6

Why Is The Sky Blue?
The sun's rays hit the Earth's atmosphere, where the light is scattered by nitrogen and oxygen molecules in the air. The blue wavelength of this light is affected more than the red and green wavelengths, causing the surrounding air to appear blue. At sunset, the sun's light passes farther through the atmosphere, deflecting and decreasing the blue in the air. Scattering by dust particles and pollution in the air causes the sunset to appear red.

Answer 7

Hooray!!! The question that has been asked more times than any other on Yahoo Answers!
There is a physical phenomenon called Rayleigh scattering that causes light to scatter when it passes through particles that have a diameter one-tenth that of the wavelength (color) of the light. Sunlight is made up of all different colors of light, but because of the elements in the atmosphere the color blue is scattered much more efficiently than the other colors.

So when you look at the sky on a clear day, you can see the sun as a bright disk. The blueness you see everywhere else is all of the atoms in the atmosphere scattering blue light toward you. (Because red light, yellow light, green light and the other colors aren't scattered nearly as well, you see the sky as blue.)

Answer 8

Yay! I know this one!

The reason the sky is blue is such: Everything on this earth reflects light, in some form or another. It just so happens, that in our universe, the oxygen molecules that we breath and that make up our ozone, O3, happen to reflect blue light. When the light passes through our atmosphere, the only wavelength of the light spectrum that doesn't penetrate the molecules of O3, is the blue spectrum. So, rather than seeing the blackness of space, we see, a blue sky.

Answer 9

The major components of the atmosphere (nitrogen, oxygen, CO2) have different light refraction wavelengths. When combined, the color that comes out is light blue. It's the way the molecules of N2, O2, and CO2 absorb and re-emit photons (particles of light). A photon of white light from the sun hits the molecule. The energy in the photon is absorbed into the molecule, causing an electron to jump up to a higher electron shell. When the electron comes back down to it's original shell, it emits another photon, of a lower wavelength. In this case, the atmospheric molecules absorb UV radiation and purple light. The photons they re-emit are the next highest wavelength, blue. That's what we see.

Answer 10

The atmosphere is composed of primarily nitrogen. Nitrogen, like any gas, has a charateristic of absorbing certain wavelengths of light. It just happens that nitrogen does NOT absorb the wavelength of light that we call "blue".

Other gases absorb other colors, but anyway... nitrogen is nitrogen.

Now, there are additional factors such as water vapor, dust particles, and the effects of pollution, etc., that affect air color, but the essential "blue" that you see is caused by what light nitrogen gas does NOT absorb.

Answer 11

Refraction of the light in the earth atmosphere. Only the blue/purple wavelength can get through the atmosphere.