Hi. Landau and Lifgarbages give an equation describing the angle of deflection of a charged particle of a given initial velocity and impact parameter. It's given on page 102 of their Classical Theory of Fields, available here: http://books.google.com/books?id=QI...X&oi=book_result&ct=result&resnum=5#PPA102,M1. It's in the solution to Problem 1, at the bottom of the page. I'm just curious where the equation in Problem 1 came from, in particular where the arctan came from. It's simple to solve 39.4 for the angle letting r-->infinity, but that gives an arccos, not an arctan. L-L just state that as the answer without working through it, as if it's obvious. Does anyone see how to derive it? Thanks!