The ray model
Physics Narrative for 11-14
Building and using the ray model of lighting
Ray diagrams are a predictive model: they are not a representation of beams of light. By following the rules for drawing the diagrams you can predict where light will travel to, and so where there will be shadows, and where will be illuminated.
These ray diagrams are also useful in predicting reflections and refractions (including lenses and prisms). More on these in the next episode.