Hi there, I was doing some past year paper of my coming exam...and I got the answers for the question too, but one part I'm totally blur and cant understand that well..... well here goes all the questions and solution: The part that i'm confused is with the part (b)...the answer given by both different book .... the solution seems.....a little blur to me. Unclear to me: 1. why "for the first time the two lights coincide, change in m = 1" ? 2. the long and madness working.....i totally have no idea what is going on... Anyone kind here could help me explain all the steps? I'm very blur... thanks in advance.