These are simple ray tracing tutorials, proceeding in stages to help learners start simple. They are designed to help provide enough infrastructure to get going right away, but not so much that the learner isn't force to experiment and try things out on their own.
It is highly encouraged to start with stage 1, and experiment with it and try to understand it. Then move on to the next stage, do the same, and so forth.
Stage 4 is, for now, the same as stage 3, and the Qt version is for those who want to have a GUI to experiment with. Stage 5 and onward will use the Qt GUI.
Stage 6 adds a BVH acceleration structure, and polygon meshes (with an OBJ reader too).
Stage 7 adds transformations (and quaternions), keyed with animation, and motion blur support, as well as a pure reflection BRDF.
Don't be afraid to break things, crash, etc. It's good for the soul!
Please see the following blog posts for more info:
http://renderspud.blogspot.com/2012/04/basic-ray-tracer-stage-1.html
http://renderspud.blogspot.com/2012/04/basic-ray-tracer-stage-2.html
http://renderspud.blogspot.com/2012/04/basic-ray-tracer-stage-3.html
http://renderspud.blogspot.com/2014/03/basic-ray-tracer-stages-4-and-5.html
http://renderspud.blogspot.com/2014/04/basic-ray-tracer-stage-6.html
http://renderspud.blogspot.com/2014/04/not-so-basic-ray-tracer-stage-7.html