Why the sky is blue

We can look up at the sky and see the blue color. Many of us spend lovely spring and summer days laying in the grass and watching the white clouds float by in the beautiful blue sky. But what makes the sky the blue color?

The sky above the clouds

To understand why the sky is blue, we have to take a look at some of the things that make it appear blue. It is just a little more complicated than a small answer. First, there is the atmosphere. The atmosphere is a mixture of the molecules (both gas and other types) that surround the earth. Depending upon where you live, the atmosphere can contain more water, near oceans or even more dust if you are near a volcano or desert.

The next thing that you need to know about is light waves. Light is energy and, depending upon the type of light, will have a variation in the size of the wave. Light is a wave of vibrating magnetic and electric fields. There are many kinds of light and each one may have a longer (or shorter) wave length. Light is just one part of a larger group that is called ‘electromagnetic fields’. The visible light (what humans can see with our eyes) is part of the electromagnetic field. It’s actually made up of a stream of colors and these are the same colors that we see in a rainbow: ROY G. BIV = Red, Orange, Yellow, Green, Blue, Indigo, Violet.

Proportion of blue light scattered

When light travels, it is in a straight line and is called ‘the speed of light’. It will continue to travel in that straight line until it bumps into something, like a drop of water or dust. The next thing that happens depends on the size of the wavelength as well as the size of whatever it bumped into. Dust and water are larger than the wavelength of light, so when it hits them it bounces of them, which is called ‘reflection’. The light goes in many different directions and at this point, the light still appears to be white because it still contains all of the same colors. Gas molecules are much smaller than the visible light wavelength. In this case, when it hits gas molecules, it acts totally differently.

When light hits gas molecules, some of the light is absorbed by the molecule. When that happens, the molecule will get ‘full’ and can begin to radiate or ‘give off’ some of the color that was absorbed. Colors of light are affected differently, even though all of the colors have been absorbed. The higher frequency colors (blues) are absorbed faster than the lower frequency colors (red). This is a scientific process that was discovered and then described in the 1870’s by an English physicist called Lord John Rayleigh. Because he discovered it, they call it ‘Rayleigh scattering’.

The daily periods of sky colours.

The reason the sky looks blue is due to the process of ‘Rayleigh scattering’. When the light goes through the air, almost none of the reds, oranges and yellow portion of the light is affected by the air. But the ‘blues’ are absorbed and reflected. You can see this when you look at the horizon in the distance. It seems like the blue is lighter in color. That’s because not enough of the blue color is being sent back or reflected to your eyes.

So, now that you know why the sky is blue, I bet you won’t look at the sky or even a rainbow the same way the next time.