Ray diagrams model events
Teaching Guidance for 11-14
The physical and the model
Wrong Track: Rays set out from the lamp and hit the lens.
Right Lines: It's beams of light that you can see spreading across the benchtop. What we draw on paper represents rays, and these are a way of predicting what the beams will do.
Using ray diagrams
Thinking about the learning
Once a ray diagram has been drawn to represent a particular event, it can then be used to predict what will happen if any aspects of that event are changed. So ray diagrams are a predictive model. They might also be used to explain. This idea of building a model that mimics the beam behaviour is a key one to get established, so that pupils know what it is they're doing and why it's important.
Thinking about the teaching
The first point to make clear to pupils is that a ray diagram is a model of a real process and drawing one involves making various simplifications. For example, a source of light might be represented as a single point and an object with a complicated three dimensional shape might be shown as a flat opaque barrier. In drawing a ray diagram we move away from the structures of the real world to represent them in a conventional, theoretical way.
Making these kinds of simplifications may seem obvious to the experienced teacher, but they are unlikely to be so obvious to pupils. For example, when asked to draw a ray diagram of how a shadow is formed by a lamp, pupils might include irrelevant details such as a person standing in a room next to the lamp.