Why is the sky blue?

The sky appears blue to the human eye due to a phenomenon known as Rayleigh scattering. This happens when sunlight enters the Earth's atmosphere and collides with gas molecules and other particles, causing the light to scatter in different directions. Blue light has a shorter wavelength than other colors of the spectrum, so it is scattered more easily than other colors. This is why the sky appears blue during the day, when the sun is high in the sky.

During sunrise and sunset, the sky appears reddish or orange due to the scattering of sunlight through the Earth's atmosphere. When the sun is near the horizon, the light must pass through more of the Earth's atmosphere, causing the shorter, blue wavelengths to scatter and leaving the longer, reddish wavelengths to dominate the sky.