The Sun is visible before actual sunrise and after actual sunset for some time. Explain this phenomenon.

(N/A) Actual sunrise is the moment the Sun crosses the horizon.
The Sun is visible a little before the actual sunrise and until a little after the actual sunset due to the refraction of light through the Earth's atmosphere. The figure illustrates the actual and apparent positions of the Sun with respect to the horizon.
The refractive index of air with respect to a vacuum is approximately $1.00029$. Due to this,the light rays from the Sun bend as they enter the atmosphere,causing an apparent shift in the Sun's position by about $0.5^{\circ}$.
The time taken for the Earth to rotate by $180^{\circ}$ is $12 \times 60 \text{ minutes}$. Therefore,the time taken for a displacement of $0.5^{\circ}$ is calculated as:
$t = \frac{12 \times 60 \times 0.5}{180} = 2 \text{ minutes}$.
Thus,the time difference between the real and apparent sunrise,as well as between the real and apparent sunset,is about $2 \text{ minutes}$.
Additionally,the oval shape of the Sun observed at the time of sunrise and sunset is also caused by atmospheric refraction.