Actually Sun is visible before Sunrise and after Sun set for some time. Explain this.

Explanation

Actual sunrise is the actual crossing of the horizon by the Sun.
The Sun is visible a little before the actual sunrise and until a little after the actual sunset due to refraction of light through the atmosphere. Figure shows the actual and apparent positions of the Sun with respect to the horizon.

The refractive index of air with respect to vacuum is 1.00029. Due to this, the apparent shift in the direction of the Sun is by about half a degree.
The corresponding time difference between actual sunset and apparent sunset is about 2 minutes.
Time taken for displacement of `180^@ = 12 xx 60` min, then time taken for displacement of `0.5^@` t = ?
`t=(12xx 60 xx 0.5)/(180)`
`therefore` t = 2 minutes
Thus, time difference between real and apparent Sunrise and real and apparent Sunset is 2 minutes.
The reason behind the oval shape of Sun observed at the time of Sunrise and Sunset is refraction.