The day is longer on the earth by about 4 minutes because:

We can see the sun before the actual sunrise by about:

Due to atmospheric refraction of sunlight, the time from sunrise to sunset is lengthened by about:

We know that light refracts (or bends) when it goes from one medium to another. Now, the atmosphere contains only air. Then how does light get refracted on passing through only air in the atmosphere?

By how much time the day would have been shorter if the earth had no atmosphere?