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**1. Homework Statement**

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. Homework 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?