Rays of light

A theoretical construct, not a tangible entity

Wrong Track: I can see the rays we need to use.

Right Lines: You select (very carefully) rays to predict what you'll see on the bench top.

Ray diagrams are based on selected rays

Thinking about the learning

Beams are what pupils see using lamps, slits and all the other apparatus on the laboratory bench top. Rays and ray drawings are used to generate models. It might help to allude to the idea of ray-tracing in computer-generated graphics as that clearly is not real, yet mimics the behaviours of the real (beams striking everyday things).

Thinking about the teaching

In any situation involving illumination, there are countless rays that might be drawn to represent light travelling from a source, striking surfaces, and so on. Drawing ray diagrams must therefore always involve the selection of key rays that enable the representation of a particular feature (for example, the extreme ends of a shadow or the position of an image).

It is a good idea to try to remember that the term ray has a special meaning in physics. It is a theoretical construction, a line on a piece of paper that we use to model real-world situations.

When describing an everyday situation, strictly speaking we should not refer to light rays travelling away from a lamp. It is sufficient simply to say that light travels away from the lamp. On the other hand, we can draw a ray diagram which predicts particular paths from the the lamp.

Teacher Tip: Remind pupils of the theoretical and modelled nature of ray diagrams. This might be reinforced by always referring to rays as drawn and not as travelling.