Why Is The Sky Blue

Before I come to the explanation as to why is the sky blue, knowing about the physical phenomenon called Rayleigh's scattering will make the task easier. This process occurs when the atmospheric particles are smaller in size than the wavelength of light which is being scattered through them. These particles may be individual atoms or molecules. This law was derived by Lord Rayleigh in 1871 and it says that the amount of scattering varies inversely as the 4th power of the wavelength. So, shorter the wavelength, the more will be the scattering of the incident light. Rayleigh scattering is more prominent when light travels through gases; although it may also occur in solid and liquids.

Reason Behind The Sky Being Blue
As mentioned, according to Rayleigh's law, the shorter the wavelength of the incident light, more will be the scattering. Now, the white light from the Sun is a combination of different colors. The visible light that we see ranges from 400 - 700 nanometers (nm) and corresponds to a color range of violet through red.

The color blue has a wavelength of about 450 - 495 nm and red has a wavelength of about 650 nm. When the Sunlight strikes the atmosphere, the gas molecules in the air (smaller than the wavelength of the light) scatter the blue light roughly about 10 times more than the red light. This is due to Rayleigh scattering wherein, light with longer wavelengths (such as the red and yellow) pass throughout the atmosphere unaffected. On the other hand, much of the shorter wavelength lights (blue and violet) are absorbed by the gas molecules and scattered in all directions. That is the reason why everywhere you look, you actually see the scattered blue light and the sky looks blue. Due to this scattering of the blue light, you see it coming not directly from the Sun but from all over the sky.

Now if this is the case, then why is the sky not violet or indigo which are of shorter wavelength than that of blue (violet being the color with the shortest visible wavelength - 400nm)? This answer can be obtained by understanding the physiology of the human eye. The human eye is known to absorb light better in the blue than in the violet or indigo shades. Technically, the sky should appear purple or violet but since the eye responds to the blue light, hence the sky appears blue. It is important to take note of that if there had been no indigo or violet, the sky would have looked blue but with a slight green tinge.

To add more to this explanation of why is the sky blue, here's why the sunset or sunrise appears red. When the Sun sets or rises, the sunlight has to travel through more of the atmosphere to reach your eyes. The light hits the earth at an angle rather directly dropping through the atmosphere (as it happens during the day). Due to the extra distance which the light has to travel, by the time it reaches the eyes, most of the blue light has been scattered out and away from the line of sight. Hence this process leaves the red or orange light unhindered through the atmosphere and reaches our eyes in a direct line with little or no scattering.

So knowing as to why is sunset or sunrise red may have helped you in deciphering as to why is the sky blue? Here is a simple experiment which kids can try out at home to see for themselves as to why is the sky blue?

Why Is The Sky Blue Experiment for Kids
Try this experiment out, it will help your kids understand it better.

Requirements
You will need:

• A clear glass jar or a drinking glass
• Water, milk powder, flashlight
• Darkened room.

Experiment
Fill the glass or jug with water (250 - 400 ml) and add about 1/2 to 1 teaspoons of the milk powder and stir slowly. Turn off the lights of the room and light the torch above the surface of the water. Observe the water in the glass from the side near to the torch. You may notice a bluish tinge. Now if you hold the light to the side of the glass and look through the water directly at the light, it will look reddish. Putting the torch under the glass will enable you see the light appearing more reddish than before.

Explanation:
The fat molecules in the water behave like the molecules in the air and as a result they scatter the light from the torch. Now, when the light shines in the top of the glass, blue light is scattered readily and a bluish tinge can be seen out of the sides. When you look through the water directly at the light, you go through more of the milk and as a result the blue light runs out due to scattering and what appears is the red color.

There are many more concepts behind the explanation as why is the sky blue? This is a simplified version of it. You can try out the experiment at home and the concept will be clearer and more conclusive. Science is as vast as the space and if you are able to unveil it, you will simply be amused and amazed! What is required is a curious mind and the quench to find the answers?..