It seems like caustics are generally more computationally heavy when it comes to rendering, especially materials with both moving reflections and refractions, such as the caustics at the bottom of the water. Here's a NVIDIA article on generating those, which mentions Snell's Law and Fresnel's equation. https://developer.nvidia.com/gpugems/gpugems/part-i-natural-effects/chapter-2-rendering-water-caustics