Phase difference in a two slit interference problem

[DODGEVIPER13]

Homework Statement

In the figure below, sources A and B emit long-range radio waves of wavelength λ = 390 m, with the phase of the emission from A ahead of that from source B by 90°. The distance rA from A to detector D is greater than the corresponding distance rB by 130 m. What is the phase difference at D? The image is at : http://www.webassign.net/hrw/hrw7_35-38.gif

Homework Equations

Δphase = 2∏L/λ(n2 - n1)
dsin(theta) = mλ

The Attempt at a Solution

Not really sure what to do I plugged 130m into L and used the given lamda and ignored n2-n1 this did not work, help me. I assume I need to use the above two slit equation but I am not sure.

 

[Steely Dan]

Homework Statement

In the figure below, sources A and B emit long-range radio waves of wavelength λ = 390 m, with the phase of the emission from A ahead of that from source B by 90°. The distance rA from A to detector D is greater than the corresponding distance rB by 130 m. What is the phase difference at D? The image is at : http://www.webassign.net/hrw/hrw7_35-38.gif

Homework Equations

Δphase = 2∏L/λ(n2 - n1)
dsin(theta) = mλ

The Attempt at a Solution

Not really sure what to do I plugged 130m into L and used the given lamda and ignored n2-n1 this did not work, help me. I assume I need to use the above two slit equation but I am not sure.

It is not generally a good strategy to ignore part of an equation in the hopes that the rest of it will be relevant to you but that part will not. Instead, think about how the double slit equation is derived: the idea is that you get a maximum brightness if the path difference to the screen from points A and B is some multiple of a full wavelength (assuming the two sources start off in phase). This problem relates to the same physical principle too. First start by answering the following: if the two waves started off in phase, would they end up in phase at point D given the information you have? Then, work out the slightly more complicated case of not starting out in phase.