CBSE Class 10 Science 2017 · Paper 31/2/1 · Question 17

Air near the Earth is denser than the air above it, and its refractive index increases gradually towards the ground. At sunrise, while the Sun is still slightly below the horizon, its light passes into denser and denser layers of air and bends continuously towards the normal, that is towards the Earth, so it reaches the observer along a curved path. The eye sees the Sun along the last direction of this light, extended backwards, which points above the horizon: the Sun appears above the horizon before it has actually risen (advance sunrise). In the same way, at sunset light from the Sun keeps reaching the observer for a short time after the Sun has actually gone below the horizon (delayed sunset). The diagram shows the actual and apparent positions of the Sun. Time difference: about 2 minutes. The Sun is seen about 2 minutes before the actual sunrise and stays visible for about 2 minutes after the actual sunset, so the day as a whole is about 4 minutes longer.