CBSE · Class 10 · Science · The Human Eye and Colourful WorldWhy does the clear sky appear blue during the day? Explain the underlying scientific principle in detail.

The clear sky appears blue during the day due to the phenomenon known as the scattering of light by molecules of air and other fine particles in the atmosphere. According to Rayleigh's scattering law, the intensity of scattered light is inversely proportional to the fourth power of its wavelength ($I \propto 1/\lambda^4$). Sunlight consists of various colours of the visible spectrum. When sunlight reaches the earth's atmosphere, the fine particles and gas molecules (such as nitrogen and oxygen) scatter light of shorter wavelengths much more efficiently than light of longer wavelengths. Blue light has a much shorter wavelength compared to red, orange, and yellow light. Consequently, blue light gets scattered in all directions by the atmospheric particles. When this scattered blue light enters our eyes, the sky appears blue to us. If the earth had no atmosphere, there would be no scattering, and the sky would have appeared dark/black to passengers flying at high altitudes, as observed by astronauts.