Walker, Halliday & Resnick — Interference: Problem 82

82 In Fig. 35-39, two isotropic point sources \( S_1 \) and \( S_2 \) emit identical light waves in phase at wavelength \( \lambda \). The sources lie at separation \( d \) on an \( x \) axis, and a light detector is moved in a circle of large radius around the midpoint between them. It detects 26 points of zero intensity, including two on the \( x \) axis, one of them to the left of the sources and the other to the right of the sources. What is the value of \( d/\lambda \)?