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Modelling light with ray diagrams - Physics narrative
Physics Narrative for 11-14
A Physics Narrative presents a storyline, showing a coherent path through a topic. The storyline developed here provides a series of coherent and rigorous explanations, while also providing insights into the teaching and learning challenges. It is aimed at teachers but at a level that could be used with students.
It is constructed from various kinds of nuggets: an introduction to the topic; sequenced expositions (comprehensive descriptions and explanations of an idea within this topic); and, sometimes optional extensions (those providing more information, and those taking you more deeply into the subject).
The ideas outlined within this subtopic include:
- Rays (not beams)
- Rays as predictive representations
Predicting what will happen using ray diagrams
One of the distinctive and interesting features of teaching and learning about light is the way in which ideas and explanations are represented with ray diagrams. Rather than just
talking through an explanation for some effect or other, you'll often find yourself constructing and referring to a ray diagram and asking pupils to
look at it this way.
This ray diagram shows the formation of a shadow by an opaque barrier. This might represent what happens when a narrow-beam torch forms the shadow of a book on a wall. This model represents the actual event in a number of ways:
- The light source is represented as a single point.
- Just two rays are shown, as straight lines drawn from the source.
- The direction of drawing for each ray is shown by an arrow.
- The book is represented as an opaque barrier.
- The position of the shadow on the wall is located between the points where the two rays meet the screen.
Drawing ray diagrams
When drawing ray diagrams, it is important that you make explicit to the pupils the steps in thinking which lie behind them. In the teaching approaches section we demonstrate how simple ray diagrams might be introduced to make that thinking clear. It's important to keep front and centre that these are special drawings for a particular purpose: there are a lot of choice to be made.
Share the choices that you make in selecting the rays that you draw.
An important feature of ray diagrams is not only that they can be used to explain phenomena, but also that they can also be used to predict. For example, in the case of the simple shadow:
- What happens to the size of the shadow as the torch is moved closer to the opaque barrier? (The shadow on the screen increases in size).
- What happens to the size of the shadow as the screen is moved away from the opaque barrier? (The shadow on the screen increases in size).
Beyond simple shadows
You can add additional rather ad-hoc rules to prescribe the drawing of the rays that produce diagrams to predict:
- refraction if there is change in medium between source and detector
- reflection, if there is a reflector between source and detector
For now, we suggest investing quite some time exploring the predictions due to rays and shadows, using simple geometrical arguments. Later the additional rules will allow you to construct ray diagrams for prisms and lenses, and later on still you may come to understand why these rules are true, and not just seemingly arbitrary rules. Its perhaps worth looking at the code to the models below to figure out how the diagrams were drawn, as that may give a preview of the rules.
There remains a real mystery here: why does the beam of light do this? That's for later: more in the SPT: Radiations and radiating topic.
What are rays?
What are rays?
An important point to be aware of here is that a ray diagram is a theoretical construction. It something that you draw for a purpose. The ray diagram is certainly not intended as a faithful reproduction of the actual real world set-up. It is a way of representing a process that includes a
point source of light and
rays – both of these are theoretical in nature.
In the real world we have light beams travelling away from lamps and some people refer to very narrow beams as pencils of light.
In the theoretical world we have rays. We draw these, often on paper, as lines from which we can predict the paths that the beams will take.
The ray model
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.