Explaining interference and path difference

We have these lovely perspex waves to illustrate double slit interference.

At the point illustrated above, a trough on the upper wave coincides with a trough on the lower wave so there will be constructive interference and a fringe will form. The bottom wave has gone two whole wavelengths less to reach this point. I think that this is a very graphic illustration of path difference. I started from the central maximum and moved out, putting dots on the table to show each fringe.