The direction they face is called the Surface Normal, and we can visualize that direction with a little 3D arrow that’s perpendicular to the polygon’s surface.
To figure it out, we use ray tracing with this simplified screen space 3D representation and bounce the rays off of the lake's pixels using the normal map.
So Bob looks at this and he wants an actual formula for that shadow, and the way he might think about it is to consider the normal vector perpendicular off of that face.