Why is the sky blue?

The sky is blue. But it’s only blue during the day. It’s dark at night. The patio is dark. So why is the sky blue during the day? Actually, the sky has no color. (It is clear like glass.)

Since the sky is blue during the day, you might think that it is related to the light during the day. It is obvious that it will be especially related to the light coming from the sun.

We all know that the sun has 7 spectrums. The spectrum seen in the rainbow, which is seen when the sun starts to rain, is the color of the sun. If all these colors are added together, they should be seen as white.

If the sun’s light is passed through a triangular glass block, the spectrum of the rainbow can be seen more clearly. (But it is very difficult to find three-pointed glass in Myanmar.)

There are gases in the earth’s atmosphere. These gas molecules can split the white light from the sun into its primary colors.

When these colors are scattered, the colors with shorter wavelengths are scattered more. Longer wavelengths can avoid passing through small gas molecules, but shorter wavelengths can’t avoid collisions.

The direction of the colliding light waves changes. It’s like billiard balls changing direction when they collide. (Here, light is made up of light particles called photons, but it has the nature of a wave, so thinking of it as a wave makes it easier to understand.)

In the spectrum from the sun, blue and violet have the shortest wavelengths, and red has the longest line lengths.

The light goes straight as usual. But if something happens in between, it becomes one of the following:

  • Reflection (objects like mirrors can reflect light)
  • Refraction (objects such as three-pointed glass and raindrops can bend light)
  • Diffusion of light (gas molecules in the atmosphere can spread light from the sun in all directions)

So when the photons of light coming from the sun collide with the oxygen and nitrogen gas molecules in the air, they are scattered around. This is called Rayleigh Scattering.

When this is spread out, the shorter wavelengths of violet and blue are more spread out. But the light coming from the sun has only a small amount of violet and more blue, so most of the light photons that are scattered are blue.

When these blue photon light particles spread across the surface, the rest of the red, yellow, and orange spectra are not very diffuse and fall directly on the earth. That’s why the color of the sun is white, but the sunlight falling on the earth is yellow, and the sun is yellow.

We see the sky as blue because of the scattered blue photons.