Two Sources, In Phase, Same Frequency: but different paths:

$\displaystyle \phi = \vert\displaystyle\frac{r_1-r_2}{\lambda }\vert 2\pi$	$\textstyle \rightarrow$	$\displaystyle \vert r_1 -r_2\vert = n\lambda \qquad {\rm constructive}$
		$\displaystyle \vert r_1-r_2\vert = (n + \frac{1}{2})\lambda \qquad {\rm destructive}$	(30)