Tell Me Why?

To explain why the sky is blue we will need to take a quick look at the Sun and the atmosphere in understanding the role they play in the sky appearing blue.

Light from the sun (sunlight) is composed of seven colors (rainbow colors) of different wavelength* that blend perfectly to make the sun’s light white in appearance. This light from the Sun is called white light. The different colors in sunlight (white light) can be seen by passing sunlight through a prism (see diagram below).

When sunlight (white light) is allowed to pass through a prism, it scatters the different colors of light according to their wavelength, showing a continuous band of colors. This procedure exhibits the range (visible light spectrum) of colors - red, orange, yellow, green, blue, indigo, and violet in that order. This means that the different colors of light have different wavelengths in which red light has the longest wavelength and blue light found at the other end of the visible light spectrum has the shortest wavelength. This band of colors appear in the same pattern as the colors of a rainbow when raindrops (acting as a prism) disperse the Sun’s white light. This means you will find that white sunlight is actually composed of the colors seen in a rainbow.

The entire planet is enveloped by a blanket of air surrounding Earth. This blanket of air is called the atmosphere, which is really a gaseous layer (mixture of gas molecules) and other materials surrounding the earth. The principal constituents of the Earth’s atmosphere are nitrogen (78 %) and oxygen (21 %). The remaining atmospheric gases in the remaining 1 percent are argon (0.9 %), carbon dioxide (0.03 %), varying amounts of water vapor (in the form of vapor, droplets and ice crystals), and trace amounts of hydrogen, ozone, methane, carbon monoxide, helium, sulfur dioxide, neon, krypton, and xenon. There are also small amounts many small solid particles such as dust, soot and ashes, pollen, and salt from the oceans.

Now, when sunlight passes through the atmosphere, it collides with the gas molecules
(oxygen and nitrogen atoms). This causes the Sun’s white light to be scattered in all directions around the sky exhibiting predominatly the blue color in the visible light spectrum much more efficiently and readily than the other colors thus making the color blue becoming visible to the eye. This is due to the fact that light with the shorter wavelengths (high frequency and higher-energy e.g. blue) are scattered much more readily than those of longer wavelengths (low frequency and lower-energy e.g. red). Therefore colors of light according to their wavelength (from the longest to the shortest wavelength) are; red, orange, yellow, green, blue, indigo, and violet. The light colors of longer wavelengths pass through the Earth’s atmosphere almost unaffected (very small scattering) while much of the shorter wavelength light is absorbed by the gas molecules which then radiate in different directions. This scattering of light in all directions is called Rayleigh scattering* and the blue color of the sky is caused by this scattering of sunlight. Even though violet has a short wavelength and is scattered readily along with blue over the sky, our eyes are much more sensitive to blue light than they are to violet light and as a result of this, we see the sky as blue.
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Side Notes
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* Wavelength: It is the distance between two consecutive peaks (crests) or troughs of an electromagnetic wave. In other words, it is the horizontal distance between two successive peaks of a wave. It is usually measured in nanometers (nm). All electromagnetic radiation (radio waves, microwaves, ultraviolet light, visible light, etc.) is transmitted in waves.

* Rayleigh Scattering: Selective scattering (i.e., preferential scattering of shorter wavelengths) of light by very small particles suspended in the Earth's atmosphere, or by molecules of the air itself. This responsible for the blue of the daytime sky.

Click image to enlarge
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