Walker, Halliday & Resnick — Interference: Problem 80

80 Figure 35-37 shows two isotropic point sources of light ($S_1$ and $S_2$) that emit in phase at wavelength 500 nm and at the same amplitude. A detection point P is shown on an x axis that extends through source $S_1$. The phase difference $\phi$ between the light arriving at point P from the two sources is to be measured as P is moved along the x axis from $x = 0$ out to $x = \infty$. The results out to $x_s = 10 \times 10^{-7}$ m are given in Fig. 35-38. On the way out to $\infty$, what is the greatest value of x at which the light arriving at P from $S_1$ is exactly out of phase with the light arriving at P from $S_2$?