1. The problem statement, all variables and given/known data Evening Physics Forums, I've been stuck on this little problem for a while which I find incredible embarrassing but here it goes. I am trying to self study optics from Born and Wolfe's text however I find myself constantly getting stuck. Here I simply try to follow line (53) to line (54) but I am confused at its explanation mathematically. I don't see how 'interchanging the independent variables and noting the imagine parts of the integrand disappear'. If someone could explain or give me an example (with a different equation, the one written above is huge lol) how this is true, it would be much appreciated. Thanks!