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Why is the Sky Blue? With Answer

Updated on August 23, 2013

A question that seem to be asked often by children is "Why is the sky blue?"

We adults don't ask these questions because we have gotten too used to this fact and so we no longer question it. Plus we have too many other weighty questions on our minds such as finance, jobs, politics, global warming, or whatever.

Nevertheless, this simple question is an important one and is one worthy of an answer. How do you answer it? Do you just tell the child that it simply is? Or do you give a more scientific answer?

Unfortunately, the answer to this simple question is not so simple afterall. Depending on the age of the child, he/she may not understand the explanation.

The Sky is Blue Due to Rayleigh scattering

In short, the sky is blue because of "Rayleigh scattering". But then that can give rise to more questions such as "Huh? What is that?"

So, a better answer might be the sky is blue due to the scattering of sunlight by gas molecules in the atmosphere.

Okay, but why the color blue? Each color of light has a particular wavelength. Blue light happens to the be wavelength that gets scattered the most that our eyes can see.

Rayleigh scattering is named after Lord Rayleigh (also known as John William Strutt, 3rd Baron Rayleigh), a British physicist who won the Nobel Prize for Physics in 1904 for the co-discovery of the element Argon.

Rayleigh scattering is the scattering of an electromagnetic wave by particles smaller than the wavelength of this wave. Sunlight is such an electromagnetic wave. Gas molecules in the sky are such particles. Sunlight is scattered by the gas molecules in the sky.

The sky is composed of particles mainly gas molecules, dirt, and water vapors. Sunlight is a combination of light of all wavelengths. It consists of the violet and blue light which has shortest wavelength, and it consists of the red light which has the longest wavelength, and it consists of all the other colors in between.

As sunlight passes through the sky, these tiny particles scatters the sunlight. The blue wavelengths of light is more profoundly affected than the other colors. Hence blue light scatters more and we see more blue in the sky.

When the sunlight hit the gas molecule, the shortest wavelengths (violet and blue) is scattered more strongly than the other colors. Violet light is actually shorter in wavelength than blue and hence is scattered more than blue. However, our eyes are not as sensitive to the violet range of light, so that is why we see mainly blue.

If we were to view from outer space (without an atmosphere), the sun would look white and the sky would look black.

Why is the Sunset Red?

A similar question is why is the sunset red? The answer is also Rayleigh scattering. It not only causes the blue hue of daytime sky, but it also causes the red hue of a setting sun.

At sunset, as the earth rotates, the sun is at a different position in the sky. The path that the light has to take to reach our eyes through the atmosphere is much longer than during the daytime. The shorter wavelengths (the violets/blues) has been entirely scattered, leaving us with the longer wavelength light (the reds).

Wikipedia says ...

"The Rayleigh scattering effect is therefore increased, radiating even more of the sun's shorter wavelength (violet and blue) light in different directions. The remaining unscattered light that is received by an observer is mostly of a longer wavelength and therefore appears to be red."

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