Microwave sources P and Q emit coherent waves that have a phase difference of $180^\circ$. Both waves share the same wavelength $\lambda$. At point S, the interference pattern formed by waves from the two sources shows a minimum. The distance $(QS - PS)$ is known as the path difference. Which expression could represent the path difference?
- A$\frac{\lambda}{4}$
- B$\frac{\lambda}{2}$
- C$\lambda$
- D$\frac{3\lambda}{2}$