The sky isn’t blue. It just appears to be blue. You might think “But the sky is clearly blue!”. And I’ll tell you again that the sky isn’t blue. You just think that it’s blue because that is easier for your brain to perceive. So your question should be: Why does the sky appear to be blue?
The Blue Sky
Our sky has a color because of our atmosphere and the light rays which come from our sun. The light rays travel through the empty vacuum of the space until they reaches the atmosphere of the Earth. When the light rays enter the atmosphere they disperse into the 7 colors of the spectrum. Then, they start hitting the tiny molecules and dust particles in the air which cause the light rays to scatter and deflect from their original paths. This effect is called “The Rayleigh Scattering Effect”
The light rays disperse into the 7 colors of the spectrum. Red has the least amount of energy and the longest wavelength while violet has the highest amount of energy and the shortest wavelength.The light rays with the shortest wavelength get scattered through all of the atmosphere. Therefore, the sky should look a mix of blue and purple but it’s not.
The reason the sky appears blue:
The reason the sky is blue and not purple lies inside your eyes. Inside your eyes there are 2 types of cells that react to light: “Rods” that are sensitive to brightness and “Cones” which are responsible for detecting color.
There are 3 types of cones inside the human eye corresponding to the 3 colors: red, green and blue. Inside your eye there are about 6 to 7 million cones and about 64% of them are red sensitive, 32% are green sensitive and about 2% are blue sensitive. However, despite blue cones having a lesser population, they have the highest sensitivity of the three types. Therefore, your eyes are more sensitive to blue than any other color.
When the light scatters in the sky, violet and blue scatter the most and reach your eyes. But, because cones are more sensitive to blue than to violet, they tell your brain that the sky is blue. And that’s the reason you see a blue sky.
The Red Sky
During the sunset the sky is orange and red. Why does it change colors from blue to red? Well, that too, has to do with the scattering of light.
When the sun is setting, it’s going downwards (from our perspective) towards the horizon. This makes it lower in the sky, and so, the distance the light has to travel in order to reach your eye is longer. The light starts scattering when it enters the atmosphere.
Due to the longer distance, blue light scatters and leaves the line of sight, before reaching your eyes. This leaves red and orange light to fill the sky. They have longer wavelengths, and scatter through the longer distance until they reach your eyes. It’s also the same for sunrises. While the sun is rising upwards from the horizon, red and orange light reach us. Then the sun becomes higher up and blue fills the sky.
If you see a gray or white sky, it’s probably because the sky above you is filled with clouds.
Written by: Osama Waheib