[vikingerik]

The sibling comment explains it too, but here's a simple description: The object still has a geometric definition, it just isn't a mesh. It has equations describing the curves. Yes, per pixel, using the curve geometry, you calculate where the ray intersects that curve, and at what angle to figure out the deflection from there.

Think of a simplification in 2D. Sunlight comes from directly vertical above a sine wave. For every x-coordinate where you cast a sun ray, you just need the slope of the sine-wave at that x-coordinate to calculate the angle for what will happen at the intersection.

A mesh isn't any fundamental unit of physics or geometry, it's just our customary way of approaching the rendering to get good-enough fast-enough results from our current hardware.