Why is the sky blue?

Quick answer:

The sky appears blue because blue light waves have a shorter wavelength and therefore scatter more than other colors of the light when they bounce off air molecules and other substances in the earth's atmosphere. Blue light scatters more quickly and more widely, and therefore, we see it more than other colors.

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Visible light is made of light of many wavelengths. The white light that we receive from the Sun is when passed through a prism separates into many colors. This is the result of light waves bending when they pass through the prism and shows that white light is made by many different colors.

When sun light passes through the atmosphere it is scattered by the particles of the gases that make up the atmosphere. Light waves with a shorter wavelength are scattered to a larger extent as compared to light waves with longer wavelengths. As the wavelength of light with the color blue is very short this is scattered the most by the air particles. Colors of rays with a longer wavelength like red are scattered to a smaller extent and pass directly through the atmosphere.

This is the reason why the sky is blue in color.

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There have been many theories about light and its components. Isaac Newton in his Particle Theory of Light, discusses the fact that white light is, in fact, composed of a series of colors. The colors separate and can be seen through a prism as individual colors. Prisms, such as those used by Newton to support his theories, make light bend. Mirrors reflect light and the atmosphere or the gases present in it, make light scatter. It is this scattering of light which is responsible for the blue sky color. 

Newton and his peers argued over light and light particles or light waves and Newton's contribution cannot be overlooked. It was however, Einstein, approximately two hundred years later, in 1911, who finally resolved the issue of light and its distribution (The Quantum Theory of Light). Einstein, from a complicated formula, calculated how light scatters from molecules. Light, a form of energy, travels in electro-magnetic waves and blue has a shorter wavelength and less mass than most other colors, except violet but this is, mostly absorbed at higher altitudes and the eyes are less sensitive to violet.

However, Lord John Rayleigh, an English physicist first described the phenomena of light scattering in 1870, as proposed by John Tyndall some years earlier, and it is the so-named Rayleigh scattering which is responsible for the blue color of the sky. Short wavelengths, such as blues, are absorbed by gas molecules (oxygen and nitrogen) in the atmosphere and then radiated out in different directions which ensures that blue reaches all directions. 

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People often wonder why the sky is black at night and blue during the day. During night time, the moon and stars are the main source of  light but during the day the sun is the main source of light. The sun gives off a lot more light than the moon does. This is the first thing to keep in mind. The second thing to keep in mind is that atoms of nitrogen and oxygen in the atmosphere have an effect on the sunlight that passes through them. A physical phenomena called Rayleigh Scattering causes light to scatter when it passes through particles that have a diameter one-tenth of the color/wavelength of light. Sunlight is made of many different colors but the sky appears to be because the color blue scattered more efficiently than the other colors. This is because blue/violet wavelengths have the shortest wavelengths. This is why the sky looks blue.

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.

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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. The sky is blue because molecules in the air scatter blue light from the sun more than they scatter red light. 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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On a bright, sunny day, the sky appears blue to us as we look up. Notice the word appears here. The sky is not actually blue, but the blue color results from the way the sunlight is scattered when it hits air molecules in the earth's atmosphere.

Sunlight looks white to us, but it is actually composed of all the colors of the spectrum or rainbow. Light is transmitted in waves, and various colors of light move in different ways. Red light, for instance, travels in long waves with peaks that are rather far apart. As we move through the color spectrum, the wavelengths of the light get shorter and shorter. Orange has shorter waves than red, yellow than orange, and green than yellow. Blue has the shortest wavelengths of all.

When light hits the atmosphere, it scatters, bouncing off air molecules, gasses, and other particles. Blue light, with its short waves, bounces more than any other color of light, and it scatters everywhere, more quickly, more intensely, and more widely. In fact, blue light is scattered in all directions about ten times more than red light is. This makes blue light more visible to our eyes. It is bouncing around all over the place, and it essentially calls our attention to itself. Therefore, the sky appears blue from that active blue light.

Notice, though, how as we look toward the horizon, we usually see more white in the sky than any particular color (at least during the day). By the time the light gets down so low in the sky, it has bounced around and scattered so much that all the colors mix back together. One does not stand out in particular anymore, and that is why we see white.

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In order to understand why the sky appears blue during the day, one must understand some general principles about light. The white light released by the sun actually contains a mix of all of the colors of light the human eye can see. This is known as the visible spectrum. This can be observed by shining light through water, which causes the light to scatter. The observer is able to see the light in its various visible colors. This is known as the Tyndall effect, first discovered by John Tyndall in 1859. 

The Tyndall effect is also why the sky appears blue during the day. During the day, the sun is at an angle relative to the viewer on the ground here on Earth. The blue light is scattered more than all the other colors. Because of this scattering by molecules in the atmosphere, when one looks up at the sky it appears blue. Interestingly, when the sun is further away relative to the viewer, such as at sunrise or sunset, the blue light is scattered so much that the observer now sees the scattering of red and orange light instead of blue.

Hope this helps!

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The sky appears blue during the day because of the scattering of light by particles in the atmosphere. The white light from the sun is made up of all of the colors of the rainbow. Blue light is scattered more than the other colors of visible light because of its shorter wavelength. We see more of the blue light because it's scattered in many directions. 

This phenomenon is called Raleigh scattering, after the British physicist Sir Raleigh. The electric field of a light wave hitting a polarizable particle causes the particle to oscillate, radiating what we see as scatted light. 

The same concept explains why the sky looks orange or red at sunset. When the sun is lower in the sky sunlight must travel a greater distance to reach us. In doing so it travels through more atmospheric gas and more of the blue light is scattered before it reaches us. The longer wavelengths, red and orange, then predominate.

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Why is the sky blue?Everytime I see the sky blue - so why is that?

The white light that our eyes see is actually made up of the colors of the rainbow.  Light travels in wavelengths.  The colors of light travel at different speeds and have different wavelengths.  This is called the spectrum.  To see the colors of light we need to have light travel through a material that can break it up such as a prism or water in the atmosphere.  The color that we see in an object is the part of the light spectrum that is most scattered or reflected back to our eyes.  In the sky the atmosphere scatters the blue light of the spectrum.  So the sky looks blue to us.  At night when there is no light to scatter, the sky looks black.  This same principle is what makes the ocean look blue to us.  If you take a glass of ocean water- the water is clear.  Because the water reflects the blue part of the light spectrum, the ocean looks blue.

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Why is the sky blue?Everytime I see the sky blue - so why is that?

Do you know what a spectrum is? These are all the colours you see in a rainbow or a prism. These colours make up the varying wavelengths of white light, only we don't see them as such unless they are broken or split up through a process called 'refraction.' 

The colour something "is" is actually the colour not absorbed by an object but reflected back instead. Which colour is absorbed or reflected back depends on its particular wavelength and that wavelength's capacity to be absorbed or reflected by a particular object.

For example, when all colours are absorbed, what you see is 'black' - nothing comes back. Along with light, some heat is also absorbed. That is why a black asphalt pavement is so steaming hot on a summer day! (You can take a thermometer and verify the difference in temperature colours make by placing it on a white piece of paper, then a black one.)

So the sky is 'blue' because the wavelength of white light most easily reflected back to Earth from the atmosphere is the one corresponding to blue. As sunset approaches, these colours change as the angle of sunlight shifts, favouring the wavelength in white light corresponding more to reddish tones.(A part of this is also due to dust in the air, but this is another factor and subject!)

Does this answer your question? In short, the 'colour' you see is actually the wavelength of white light rejected by a particular surface, kind of like a ball bouncing back from off an invisible wall instead of passing right through.

Check out the references below for more information, and another experiment you can do.

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