If shorter wavelengths are scattered most strongly, then there is a puzzle as to why the sky does not appear violet, the colour with the shortest visible wavelength. The spectrum of light emission from the sun is not constant at all wavelengths, and additionally is absorbed by the high atmosphere, so there is less violet in the light. Our eyes are also less sensitive to violet. That's part of the answer; yet a rainbow shows that there remains a significant amount of visible light coloured indigo and violet beyond the blue. The rest of the answer to this puzzle lies in the way our vision works. We have three types of colour receptors, or cones, in our retina. They are called red, blue and green because they respond most strongly to light at those wavelengths. As they are stimulated in different proportions, our visual system constructs the colours we see.

Why does the sky appear blue and not violet?

The phenomenon of the sky appearing blue rather than violet is primarily due to the scattering of sunlight by the Earth's atmosphere. Sunlight is composed of different colors with varying wavelengths, ranging from violet to red. However, certain factors influence the dominant color observed in the sky, with blue being the most prominent.

Scattering of Light
When sunlight enters the Earth's atmosphere, it encounters tiny molecules and particles present in the air. These particles can scatter light in different directions. The scattering process is influenced by the wavelength of light, with shorter wavelengths being scattered more easily.

Rayleigh Scattering
The primary scattering mechanism responsible for the blue color of the sky is known as Rayleigh scattering. This type of scattering is more effective for shorter wavelengths, such as blue and violet light. As sunlight enters the atmosphere, the shorter blue wavelengths are scattered more strongly compared to longer wavelengths like red, which are relatively unaffected.

Path Length and Scattering
The path length of sunlight through the Earth's atmosphere also plays a role in the observed color of the sky. The shorter blue wavelengths are scattered multiple times as they travel through the atmosphere, increasing the chances of encountering scattering particles. This leads to a greater overall scattering of blue light compared to violet light, resulting in the dominant blue color of the sky.

Sensitivity of Human Vision
Another factor contributing to the perception of a blue sky is the sensitivity of human vision. Our eyes are more receptive to blue light compared to violet light. The cones in our retinas are more responsive to blue wavelengths, making it easier for us to perceive the blue color of the scattered light.

Summary
In summary, the sky appears blue instead of violet due to Rayleigh scattering, where shorter blue wavelengths are scattered more efficiently by the Earth's atmosphere compared to longer wavelengths. The path length of sunlight and the sensitivity of human vision further contribute to the dominance of the blue color.