Why is the sky blue?

Answer

The sky is blue due to the scattering of sunlight, also called Rayleigh scattering with a difficult word. This has to do with the way in which the light from the sun reaches us through space and atmosphere.
Sunlight is white, but this white light is actually a mixture of different colors: red, orange, yellow, green, blue, indigo and violet. These are also the colors you see in a rainbow. Each of those colors can be described by a wave of a certain wavelength. The blue color has a shorter wavelength than the red color.
Before sunlight reaches the Earth’s surface, it must pass through the atmosphere. For the long wavelength colors like red this is easy, but for the short wavelength colors like blue it is more difficult. Red light travels almost straight ahead, but blue light is sent in different directions due to the presence of particles (gas molecules such as nitrogen, oxygen) in the atmosphere and this is called Rayleigh scattering. Because the blue color of the white sunlight is scattered much more than the red color of the white sunlight, the sky looks blue.
The larger the particles in the air, the more scattering there is. So if there are all kinds of other particles in the air in addition to gas molecules, other colors such as red light can also be scattered. If, in addition to the blue light, the red light and yellow light are also scattered, the sky will no longer see blue, but rather light blue or even white.
Take a look outside if it is a beautiful sunny day with clean air (and therefore few particles in the air), then the sky will look clear blue. If you look outside on a rainy and humid day (and therefore many particles in the air), the sky will turn white (gray). .
If there were no scattering (i.e. no atmosphere and no particles in the air) we would see the sky as pitch black.