Tổng hợp dao động tại M: \(u_{M}=2acos(\pi \frac{d_{1}-d{2}}{\lambda }-\frac{\varphi }{2})cos(\omega t+\frac{d_{1}+d_{2}}{\lambda }\pi +\frac{\varphi }{2})\)
Mặt khác: \(d_{1}-d_{2}=IA+\frac{\lambda }{3}-(IB-\frac{\lambda }{3})=\frac{2\lambda }{3}\)
\(\Rightarrow u_{M}=2acos(\pi \frac{\frac{2\lambda }{3}}{\lambda }-\frac{\varphi }{2})cos(\omega t+\frac{d_{1}+d_{2}}{\lambda }\pi +\frac{\varphi }{2})\)
Vì M cực tiểu: \(\Rightarrow \pi \frac{\frac{2\lambda }{3}}{\lambda }-\frac{\varphi }{2}=\pm\frac{\pi }{2}\)
\(\Rightarrow \varphi =\frac{\pi }{3} \) hoặc \(\varphi =\frac{7\pi }{3}\)
\(\Rightarrow \varphi =\frac{\pi }{3}\) (Vì \(0\leq \varphi \leq 2\pi\) )
