Walker, Halliday & Resnick — Interference: Problem 79

79 In Fig. 35-37, two isotropic point sources of light ($S_1$ and $S_2$) are separated by distance $2.70\,\mu\text{m}$ along a y axis and emit in phase at wavelength $900\,\text{nm}$ and at the same amplitude. A light detector is located at point $P$ at coordinate $x_P$ on the $x$ axis. What are (a) the greatest value of $x_P$ and (b) the second greatest value at which the detected light is minimum due to destructive interference?