The Wikipedia page for the double-slit wave interference experiment states that the path difference between waves diffracting from the two slits is equal to:
$$ \Delta l=d\sin \theta $$
where $d$ is the distance between the two slits and $\theta$ is the angle defined in the diagram on the same page (between the line bisecting the two slits and the point of interference).
Does anyone know how this path difference is derived? It seems like it should be a fairly simple geometric relationship, but I can't figure it out.