a study of HDR (High Dynamic Range Rendering), that is, e.g., given real-valued numbers instead of integers for the color components you have a greater range of pixel-luminances. this allows further techniques like tone-mapping to be applied. if you have a sensible look at the different versions of the rendering you will recognize that only one global scaling factor will lead to missing details in the image.
a study of a closed-ring caustic, where light can't escape, and one of a half-ring, where most of the light escapes. the closed-ring image made me think about the magic of equilibrium, it's really highly fascinating.