Why does sun is set 4minutes …

When the light rays passes through the atmosphere having layers of different densities and refractive indices, then atmospheric refractioon takes place. The actual sunrise takes place when the sun is above horizon. When the sun is just below the horizon, the light rays coming from it , on entring the Earth's atmosphere suffer atmospheric refraction from a rarer to denser medium . so, they bend towards the normal at each refraction. Due to continuous refraction of light rays at each layer, it follows a curved path and reaches the eye of the observer. As a result , we can see the sun 2 mins. before it rises above the horizon in the morning .

It is also due to refraction that we can still see the sun for abt 2 mins. even after the sun has set below the horizon.

As a result, the time from sunrise to sunset is increased by abt 4 mins.