Question

In my physics book it says, The radiation that reached the Earth has to go through the are of a disc of Radius R ( that of the earth)... making it Pi*R^2, i don't get why that is true?

Answer 1

this may just be a commentary on just how little of the earth's radiation gets to us. look at the 2 scales. that part of the earth's energy that gets to us has passed down a tiny little tube, from sun to earth, that has radius which is same as same as earth's radius.