CBSE Class 10 Science 2017 · Paper 31/2/C · Question 10

The phenomenon is atmospheric refraction (the refraction of sunlight by the Earth’s atmosphere). Reason: the air near the Earth’s surface is denser than the air higher up, so the refractive index of the atmosphere increases gradually towards the ground. When the Sun is still slightly below the horizon, its light enters the atmosphere and passes from rarer into denser layers, bending towards the normal at every layer, so its path curves towards the Earth and reaches the observer. The observer sees the Sun along the direction from which the light finally enters the eye, and that direction is above the horizon. So the Sun appears above the horizon about 2 minutes before the actual sunrise (its actual crossing of the horizon). In the same way, at sunset its light is still bent towards us after it has gone below the horizon, so it stays visible for about 2 minutes after the actual sunset. Diagram: the observer on the Earth, the horizon, the actual Sun below the horizon, the ray bending in the layers of air, and the apparent Sun above the horizon on the line of sight.