Question

Why the sky is blue and why an astronaut sees sky black always?

Answer 1

This is due to scattering of light...
Hope it helps you dear...

Answer 2

The molecules of air and other fine particles in the atmosphere have size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue and than light of longer wavelengths at the red end. The red light has a wavelength about 1.8 times greater than the blue light. Thus, when sunlight passes through the atmosphere, the fine particles in air scatter the blue colour ( shorter wavelengths) more strongly than red. The scattered blue light enters our eyes. If the earth had no atmosphere,there would not have been any scattering. Then, the sky would have looked dark. The sky appears dark to astronauts flying at very high altitudes, as scattering is not prominent at such heights.