The Sun is visible to us two minutes before the sunrise and two minutes after the sunset because of the bending of the light due to atmospheric refraction. The actual sunrise takes place when the Sun is just above the horizon. But due to atmospheric refraction of light, we see the Sun about two minutes before it is actually above the horizon.
When the Sun is slightly below the horizon, its light moves from less dense air to more dense air and gets refracted towards the normal. Because of this atmospheric refraction, the Sun appears to be above the horizon when it is actually slightly below the horizon. It is also due to atmospheric refraction that we can still see the Sun for about two minutes even after it has set below the horizon.
