1. The problem statement, all variables and given/known data The more I think about path difference, the more confused I get. First of all, HOW DO PEOPLE know that the path difference is dsinθ? Why do we draw a line coming from the first ray that is PERPENDICULAR to the second ray when determining path difference? Please see diagram: If the PD is the extra distance that ray B travels, then subtracting the PD would leave AP=OP, so now you have an isosceles triangle, and the two blue angles must equal. Now, if we do "dsinθ" to determine the PD, then that gives us a right triangle. If the orange angle is always 90 degrees, then the blue angles must also be 90 degrees. This doesn't make sense, because this suggests that the two rays are parallel. If they are parallel, they'll never converge to the same point and interfere. So my question is: is this "dsinθ" formula for PD simply a simplified assumption because the slits are so close together and the screen is so far away that the rays can be thought of as "nearly parallel"? But strictly speaking the orange angle is NOT actually 90 degrees in reality? Please help. Thanks!