Why is the sky blue?


Photo by Zoltan Tasi on Unsplash

Why is the sky blue while the clouds aren’t? This may seem like a difficult question at first, but the physics behind this isn’t difficult at all.

There are two things that play an important role in giving color to the sky:

  1. The atmosphere of the earth
  2. Sunlight

The light rays coming from the sun are white in color. This white light is a mixture of the seven colors of the rainbow. These colors are violet, indigo, blue, green, yellow, orange, and red. Each color of this light has a wavelength associated with it. The red-colored light has the longest wavelength and the violet-colored light has the shortest wavelength of them all.

The earth’s atmosphere comprises mostly atmospheric gases, dust particles, water droplets, and water vapor. The atmospheric gases include nitrogen (78%), oxygen (21%), and other gases like argon, carbon dioxide, etc.

When sunlight falls on the air molecules in the atmosphere, it gets scattered. Scattering is the process where light rays are absorbed by the sunlight and re-emitted in different directions.

Rayleigh’s law says that the lesser the wavelength more will be the scattering. Blue, violet, and indigo will be scattered most by these molecules in the air.

When this light appears to our eyes, the sky appears blue to us. The sun emits more blueish light compared to violet hence the sky does not appear violet to us.

Why aren't the clouds blue?

This is because a cloud is made up of millions of relatively large water droplets. These droplets scatter all colors almost equally. This means that the sunlight continues to remain white.

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