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

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

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?

A student claims that because of atmospheric refraction, the sun can be seen after it has set, and the day is, therefore, longer than if the earth had no atmosphere.

(a) What does the student mean by saying that the sun can be seen after it has set?

(b) Do you think that the students' conclusion is correct?

What is the colour of the sunlight:

(a) scattered by the dust particles in the atmosphere?

(b) scattered by the air molecules in the atmosphere?