The sky appears blue due to a phenomenon called Rayleigh scattering. It occurs when sunlight interacts with the Earth's atmosphere, specifically with the molecules and small particles present in the air.
Sunlight consists of various colors of light, each with different wavelengths. These wavelengths range from shorter blue and violet wavelengths to longer red and orange wavelengths. When sunlight enters the atmosphere, it encounters molecules such as nitrogen and oxygen, as well as tiny particles like dust and water droplets.
The size of these particles and molecules is smaller than the wavelengths of visible light. When sunlight interacts with them, the particles scatter the light in all directions. However, the scattering of shorter blue and violet wavelengths is more prominent than that of longer red and orange wavelengths due to the size of the particles and the physics involved.
The blue and violet wavelengths of light are scattered more efficiently in all directions by the molecules and particles in the atmosphere. As a result, when we look up at the sky, our eyes receive a significant amount of this scattered blue light from all regions of the sky. This scattered blue light is what we perceive as the blue color of the sky.
Additionally, our eyes are more sensitive to blue light compared to violet light, which further contributes to our perception of the sky as blue.
It's important to note that the sky can appear different colors under certain conditions. For example, during sunrise or sunset, the sky often appears reddish or orange due to a different scattering phenomenon called Mie scattering. This occurs when the sunlight has to pass through a thicker portion of the atmosphere, causing the scattering of longer wavelengths to dominate.
In summary, the sky appears blue because of Rayleigh scattering, which causes shorter blue and violet wavelengths of sunlight to scatter more efficiently in the Earth's atmosphere, leading to our perception of a blue sky.