Understanding the Phases in the Double-Slit Experiment

[I Am Robot]

1. Homework Statement and all the rest

This is from a take home exam we already had graded. Professor does not mark our papers or review and we have no tutors. :(

Can someone point where I went wrong? The blues are the few markings he made that I went over again and the orange just seperates.

 

[vela]

Sorry, but it's not so much a matter of where you went wrong; you never even got started. First, you didn't do the problem the way you were ask to, using phasors. Second, the answer you got, 3 E₀, is physically impossible, and you should have recognized this. Finally, it's pretty clear you don't understand the basic concepts behind the double-slit experiment.

Start by answering the question: Why do the waves from the two slits arrive at a point on the screen with different phases?