We don't feel comfortable with slits yet. It's difficult to visualize the path length difference, and I think I spent too much time lecturing. I'll fix it next week.
Still, we did get to see how two slits work in the real world. And, when we went to the diffraction grating, which is just an infinite number of slits to a physicist, we could predict what would happen. I'll take that.
It's interesting to watch the patterns, both good and bad, we've fallen into as a class. The patterns are mostly good; don't get me wrong. But I like hearing the shared language, the statements of "this fits with our model" or "wait, is this like.." as we tried to find connections between our models. It's a great feeling when you feel you've changed how students conceptualize science.