Question

why sky is blue in the day and black in the night?​

Answer 1

The daytime sky is blue because light from the nearby Sun hits molecules in the Earth's atmosphere and scatters off in all directions. At night, when that part of Earth is facing away from the Sun, space looks black because there is no nearby bright source of light, like the Sun, to be scattered.

The sunlight reaching our eyes has a high ratio of short, bluish wavelengths compared to medium and long wavelengths, so we perceive the sky as being blue. Without an atmosphere the sky appears black, as evidenced by the lunar sky in pictures taken from the moon. But even a black sky has some lightness.

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Answer 2

The daytime sky is blue because light from the nearby Sun hits molecules in the Earth's atmosphere and scatters off in all directions. The blue color of the sky is a result of this scattering process. At night, when that part of Earth is facing away from the Sun, space looks black because there is no nearby bright source of light, like the Sun, to be scattered. If you were on the Moon, which has no atmosphere, the sky would be black both night and day. You can see this in photographs taken during the Apollo Moon landings.