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## 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 im not sure.