Why is the sky blue?

The sky appears blue due to a phenomenon known as Rayleigh scattering, which involves the interaction between sunlight, the Earth's atmosphere, and the human eye.

Here are the key points:

Interaction with the Atmosphere

When sunlight, which is white light containing all the colors of the visible spectrum, enters Earth's atmosphere, it encounters tiny molecules of gases such as nitrogen and oxygen. These molecules are much smaller than the wavelengths of visible light235.

Scattering of Light

The shorter wavelengths of light, particularly blue and violet, are more likely to be scattered by these small molecules. This scattering is known as Rayleigh scattering, named after Lord Rayleigh who first described the phenomenon. Blue light, having a shorter wavelength than red light, is scattered more intensely in all directions235.

Human Vision

Although violet light has an even shorter wavelength and is scattered more than blue light, our eyes are less sensitive to violet light. This, combined with the fact that the sun emits more blue light than violet light, means that we perceive the sky as blue rather than violet135.

Sunlight and Atmospheric Path

During the day, when the sun is overhead, the light travels a shorter distance through the atmosphere, and the blue light is scattered in all directions, making the sky appear blue. At sunrise and sunset, the sun's light has to travel through more of the atmosphere, scattering away most of the blue light and leaving the longer wavelengths of red, orange, and yellow to reach our eyes, resulting in the reddish hues of these times135.

In summary, the sky appears blue because of the preferential scattering of blue light by the small molecules in the Earth's atmosphere and the relative sensitivity of human eyes to different wavelengths of light.