$(a)$ With the help of a suitable diagram,explain why the Sun is visible to us two minutes before the actual sunrise and two minutes after the actual sunset.
$(b)$ Name the phenomenon responsible for the apparent flattening of the Sun's disc at sunrise and sunset.

(N/A) The Sun is visible to us about two minutes before the actual sunrise and about two minutes after the actual sunset because of atmospheric refraction.
By actual sunrise,we mean the actual crossing of the horizon by the Sun.
As light from the Sun enters the Earth's atmosphere,it undergoes refraction due to the varying density of air layers.
This causes the light rays to bend towards the normal,making the Sun appear slightly higher than its actual position when it is just below the horizon.
The time difference between the actual sunset and the apparent sunset is about two minutes.
$(b)$ The phenomenon responsible for the apparent flattening of the Sun's disc at sunrise and sunset is also atmospheric refraction.