Walker, Halliday & Resnick — Interference: Problem 67

67 In Fig. 35-35, sources \(A\) and \(B\) emit long-range radio waves of wavelength \(400 \text{ m}\), with the phase of the emission from \(A\) ahead of that from source \(B\) by \(90.0^\circ\). The distance \(r_A\) from \(A\) to detector \(D\) is greater than the corresponding distance \(r_B\) by \(150 \text{ m}\). What is the phase difference of the waves at \(D\)?