The Basics...

The RayTrace Applet helps you visualize what happens when a raytracer fires a ray into a scene.  It consists of 2 visualization panes, and two controls.  The left hand pane is the Scene pane, and on the right is the Trace pane.  

The Trace Pane

This pane presents a pre-rendered image of the scene.  An offline raytracer program creates a bunch of images of the scene, at various recursive depths, and saves them.  This pane simply loads the proper image, so you can see what the traced image looks like.

When you click on the Trace Pane image, a ray is traced through that point in the Scene pane.

You can control which image is loaded by using...

The Controls

There are two controls: a depth slider and a drop-down box.  The drop-down box lets you choose how the recursive tracing is terminated, either through a fixed depth, or by adaptive termination based on ray contribution.  The slider controls whichever method is selected, letting you choose the depth of recursion, or the limiting percent at which rays are no longer traced.  At 100%, a reflection or refraction ray is only fired if its attenuation (due to the object's reflection coefficient, and the accumulation of these as more rays are fired) is 1.0, that is, it will contribute fully to the pixel where it originated (this almost never happens, so this is similar to a recursive depth of 0).  At 0%, a reflection ray is always fired (to a max depth of 20).

The Scene Pane

What you see:

When you first start the applet, this pane shows you a view of the raytraced scene as seen from some arbitrary point.  The Center of Projection of the raytracer is shown as a yellow dot, and the viewing frustum is a yellow-outlined pyramid.  At the end of this pyramid is the image plane, where the image from the trace plane shows up.  Point lights appear as colored balls, and transparent objects are hard to see because they are transparent, without any reflection or refraction effects.

Firing rays:

When you click (or click and drag) on the Trace Pane, a ray is drawn in the Scene Pane from the center of projection, through the image plane, and into the scene.  When it hits something, it fires rays for reflection, refraction, and light rays toward each light.  The light rays are colored to match the light they hit, while a yellow ray means it didn't hit anything, and a dotted ray is a reflection or refraction.

Controlling the scene:

To move the scene around so you can see the effects more clearly, a 3-button mouse is best.  If you don't have one, the same effects can be achieved using the alt and meta keys, but I'm not sure which does what.  But mouse control: Dragging with the left button rotates the scene in 3-D in this funky trackball fashion that can be kind of disorienting.  Usually, your scene will wind up tilted from vertical in some strange way.  To solve this, you use the tilt control, which is an invisible horizontal area across the top of the image.  Left-clicking near the top of the image is for tilt rather than rotate.  The middle button zooms.  The right button pans the scene, which is also a bit weird.  Be careful about getting lost under the base plane of the image.