To determinate the color of the original point in the viewing plane, this set of rays must be traced backward trough the scene. A simple example demonstrating recursive ray tracing. There we have two types of illumination - direct and global. Direct illumination is light incident on a scene from a light source or a number of light sources. A surface receives light directly from a source.
Global illumination represent the light with origin of source light incident on a surface element.
Ambient term is there modeled as a constant. A ray striking a partially or wholly transparent object is refracted owing to the change in the velocity of light in different media. If a ray is traveling from a more to a less dense medium then it is possible for the refracted ray to be parallel to the surface. In this case incident angle is called as the critical angle. If incident angle increased over critical angle, total reflection occurs.