Why has the sky most often a blue color? And for what reason becomes the sky becomes suddenly reddish when the sun sets? Why does this red color only appear during a sunset or sunrise and not when it is noon?

Light that originates from the sun consists of different colors: red, orange, yellow, green and blue. These colors can be found in for example a rainbow. The only difference between blue and red light is the wavelength; blue light has a shorter wavelength than red light.

If you to an object, for example a red book, you will see this book red because only red light is reflected. The other colors (wavelengths) are absorbed by the book. Something similar takes place in the sky. The air contains mainly oxygen and nitrogen molecules. These molecules are very tiny particles. The shorter the wavelength, the better it feels the molecules. So because blue light has a shorter wavelength than red light, blue light is scattered (or reflected) more by the air molecules than red light.

If you are looking to the sky, the light that enters your eye is scattered by the air molecules, so you will see blue light.

During a sunset or a sunrise this goes opposite. Then, if you are looking in the direction of the sun, you will see mainly light that is not reflected, but transmitted. That is primarily red light, so therefore the sky is colored red during a sunset or sunrise.

During the day you will not see this red color. This is because the sun is standing higher in the sky, therefore the light rays are passing a shorter distance through the sky before entering your eye. As a result the effect of the scattering is smaller, such that you do not see that the air around the sun is slightly more reddish.