A common (if somewhat impractical) technique used to demonstrate how powerful shaders can be is to use them to ray trace a scene. Thus far, all of our rendering has been done with polygon rasterization, which is the technical term for the triangle-based rendering that WebGL incorporates. Ray tracing is an alternate rendering technique that traces the path of light through a scene as it interacts with mathematically defined geometry.
Ray tracing has several advantages compared to traditional polygonal rendering. Primarily, this includes creating more realistic scenes due to a more accurate lighting model that can easily account for things like reflection and reflected lighting. That said, ray tracing tends to be considerably slower than polygonal rendering, which is the reason it's not often used for real-time applications.
Ray tracing a scene...