CBSE Class 10 Science 2018 · Paper 31(B)/C · Question 12

Why the sky appears blue from the Earth: the molecules of air and other very fine particles in the atmosphere are smaller than the wavelength of visible light. Such particles scatter light of shorter wavelength (blue) much more strongly than light of longer wavelength (red); red light has a wavelength about $1.8$ times that of blue light. So when sunlight passes through the atmosphere, its blue component is scattered in all directions over the sky, and this scattered blue light enters the eyes of an observer on the Earth from every part of the sky. Hence the sky appears blue. For the astronaut: the sky appears dark (black). The International Space Station orbits far above the atmosphere (about $400$ km up), where there is practically no air. With no particles to scatter sunlight, no scattered light reaches the astronaut's eyes from the sky, so the sky looks black even in daylight.