Ray Diagrams
Light, Sound and Waves

Interpreting ray diagrams

Teaching Guidance for 14-16 Supporting Physics Teaching

Ray diagrams as a theoretical tool

A ray diagram is a tool used by physicists to explain or predict the behaviour of beams of light as they pass through objects such as glass blocks or lenses. When ray diagrams are first introduced, students not surprisingly often assume that they show the world as it really is. This is not the case. Ray diagrams belong firmly to the world of theory – to a world where rays of light travel out from a point source, and where single rays are refracted and reflected in particular ways in relation to normal line constructions.

In the real world beams of light exist (like the beam from a car headlamp) but point sources and normal lines do not.

Ray diagrams for explaining a whole range of phenomena

The distinction between real and theoretical worlds in relation to light is important. It is worth emphasising in your teaching:

Teacher: So, ray diagrams offer us a really important tool for dealing with a whole range of phenomena related to light. These include how we see, how telescopes work, and how rainbows are made. Just as we can use a hammer to work on a range of DIY jobs, so we can use ray diagrams to explain a variety of effects. When we use a telescope, there are no individual light rays being refracted through the lenses, but these simplified ray diagrams help us understand how the telescope works.

Learning how to draw ray diagrams for particular cases, such as the reflection of light in a mirror, the refraction of light through glass blocks, and the refraction of light in lenses, is just a starting point. Students need help in moving between the world of theory and the real world. The core message here is that learning to construct ray diagrams is not an end in itself; students need practice in applying them to real world situations. Otherwise what is the point?

Teacher: OK, so we've got this diagram that shows a ray of light being refracted away from the normal as it passes from water to air. Nice diagram. Normal line clearly shown. But what has this to do with the swimming pool or river seeming shallower than it really is? Who can use the ray diagram to help explain?

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