1. The problem statement, all variables and given/known data http://i.imgur.com/FiYb9OE.png Problem b). Looking at the attached document of my teacher, the visual representation of the answer does make sense. 2. Relevant equations 2pi(delta(x1-x2))+delta(phase constant) Basic interference problem in One-Dimension. 3. The attempt at a solution After all, the two waves end up contructively interfering. However, in the picture, the phase constant of 2 is pi/2 and the phase constant of 1 is 0. My teacher uses the equation 2pi(delta(x1-x2))+delta(phase constant). However, here is what I do not understand. For delta(phase constant), she substracts as follows: phase constant of 2 (pi/2) minus phase constant of 1 (0). Inevitably, she gets a positive value for delta(phase constant). Why is it that she can change the order of the delta? "1"-"2" vs "2"-"1" I looked at the proof of this formula to dig up more info. For those interested, I looked up chapter 21.6 of Knight's Physics textbook. The author clearly mentions that when one chooses a particular order, he must stick to it for both delta x and delta phase constant. I tried sticking to it. But the result gives me destructive interference! Why?