Last year, I watched a lot of video of students working through worksheets on topic of light and shadow. Their questions led me to think about this problem:
The worksheets had students think about this situation: a mask with a circular hole, a screen, and a small (i.e., "point source") bulb are arranged like this
Students are supposed to think about "where" the light goes when you raise it. But one group went about thinking about what would happen to the shape and size of the spot of light as you move it from aligned with the hole to farther and farther up or down? Students questions are the best ones.
So, How does the size/shape of the bulb change as you move the bulb up and down? First give your intuitive guess... and then do some work to figure it out and convince someone else.
(I have three very different approaches to answering this. My most elegant came just last night as I was falling asleep.)