• Purpose of Ray Tracing
• Setup
• How Ray Tracing Works

Purpose of Ray Tracing
The purpose of ray tracing is to recreate photo-realistic 3D images on a 2D computer screen.  This promising computer graphics technology simulates light rays within a 3D environment.  Since light rays have predictable physical properties, the ray tracing algorithm attempts to calculate the exact coloring of each ray-object intersection.  Ray tracing is revolutionary because it allows light rays to bounce from object to object, which is what they do in real life.  Ray tracing is by far the most advanced computer graphics technology.
 

Left image Courtesy of IEEE Computer Grphics and Application, issue 3, 1995. page: 48
Right image Courtesy of IEEE Comp. Graphics and App. issue 3, 1995. page: 49.

Setup
The basic set-up of ray tracing includes an eye, a viewplane, a model, and a light source.  The model is a collection of 3D objects (i.e. spheres, cubes, polygons.)  This model is illuminated by a light source that radiates light uniformly in all directions.  The model and the light source make up a 3D "scene."  The ray tracing algorithm views the scene from a designated point in space called the eye, through a rectangular window called the viewplane.  Each point on the viewplane has a direct correspondence with a pixel on the computer monitor screen.  By simulating light rays in the scene, the computer figures out the exact coloring of each pixel on the screen and displays the 3D model, with all of its optical effects, on the 2D screen.
 
Image Courtesy of Byte Magazine, issue 1, 1997. page: 264.

How Ray Tracing Works
The ray tracer first models the scene using geometric primitives (often polygons).  In this phase, the algorithm creates objects, defines the eye and the lights, and determines how each object will look.  This first stage in ray tracing is called modeling.  After the algorithm finishes its calculations, it creates the images out of the resulting geometric description by applying surface characteristics to the objects.  This stage is called rendering.
 
These two images Courtesy of Byte Magazine, issue 13, 1990. page 254.