A swimming pool is 2.7 m deep. How deep does it seem when you look straight down into it?

A ray through a glass slab leaves parallel to the way it came ($e = i$), shifted sideways by the lateral displacement $d = \dfrac{t \sin(i - r)}{\cos r}$. Looking straight down into water, $\dfrac{\text{real depth}}{\text{apparent depth}} = n$: things look shallower.