The sky appears blue due to a phenomenon called Rayleigh scattering. Sunlight consists of many colors, each with different wavelengths. As sunlight enters Earth’s atmosphere, it interacts with gas molecules and small particles. Colors with shorter wavelengths, like blue and violet, scatter more than those with longer wavelengths, like red and yellow.
Even though violet light scatters even more than blue, our eyes are more sensitive to blue, and the upper atmosphere absorbs much of the violet light. As a result, we primarily see a blue sky【22†source】【23†source】.
This scattering process, first explained by Lord Rayleigh, is influenced by the size of the particles in the atmosphere being much smaller than the wavelength of visible light【24†source】.