Why is the sun visible before Sunrise and after Sunset?
Answer: The sun is seen before the sunrise and after the sunset due to the refraction of light by the atmosphere. Air density is highest at the surface under normal conditions. Air density decreases as you go up. Even when the sun is below the horizon during sunrise and sunset, light rays from the sun are refracted from denser to lighter layers of air.
As a result, the refracted rays at each layer are bent to the perpendicular and reach the viewer’s eye at the surface in a curved path. The viewer’s eye cannot follow the curved path. So they see the rays straight. As a result, the apparent position of the Sun is slightly above its actual position. So we see the sun a little before sunrise and a little after sunset even when the sun is below the horizon.