i. The sky appears dark to the astronaut because in space there is no atmosphere which contains fine particles and air molecules and therefore there is no scattering of light. We are able to see the sky blue due to scattering of light by the fine particles and air molecules present in the atmosphere. Scattering phenomenon occurs only when the size of the atom is comparable to the wavelength of light.

ii. During sunrise or sunset, the sun is close to horizon and therefore the light rays coming from the sun have to travel larger part of the atmosphere. As the light travels through the atmosphere, all the other colours except red get scattered away by the air molecules and fine particles present in the atmosphere. Red colour has the longest wavelength and hence is less scattered and therefore reaches our eyes.

iii. Stars appear to twinkle because stars are very far away from the earth and emit their own light. As they are very far they are considered as point objects. The light coming from stars has to go through different density or layers of earth’s atmosphere and therefore is refracted multiple times. So sometimes the refracted light has more intensity therefore stars appear brighter and sometimes the refracted light has less intensity which makes the star dull. This causes the twinkling effect.