Bringing together two sets of constraints

Learners: Distinguishing–eliciting–connecting

How to:

• separate reflection and refraction
• avoid restricting the idea of reflection to shiny surfaces
• not conflate beams with rays
• reinforce the role of reflection in seeing
• connect seeing to both specular and diffuse reflections
• show where reflection and refraction occur outside the school laboratory

Teacher Tip: These are all related to findings about children's ideas from research. The teaching activities will provide some suggestions. So will colleagues, near and far.

Focusing on the physics:

Representing–noticing–recording. How to:

• use ray diagrams to account for reflections
• use ray diagrams to account for refraction
• distinguish between shadows and rays
• show clear examples of the phenomena
• introduce a wide range of surfaces from which reflection happens
• introduce a wide range of changes of medium at which refraction happens

Teacher Tip: Connecting what is experienced with what is written and drawn is essential to making sense of the connections between the theoretical world of physics and the lived-in world of the children. Don't forget to exemplify this action.

All surfaces reflect

Wrong Track: You only get reflection of light with mirrors.

Right Lines: Reflection of light occurs at any surface, such that the angle of incidence is equal to the angle of reflection, and not just mirrors.

Starting with mirrors

Thinking about the learning

Many pupils come to associate reflection and the law of reflection of light solely with the action of plane mirrors. They are often unaware that reflection (with the angle of incidence equal to the angle of reflection) occurs when light meets any surface.

Thinking about the teaching

Start your teaching with the reflection of light from plane mirrors to establish the law of reflection, then move on to diffuse reflection from irregular surfaces. Emphasise that the law of reflection of individual rays of light applies to both regular and diffuse reflection.

Refraction is sometimes referred to as bending light.

Not really objectionable

Teacher Tip: You might want to distinguish between bending a ruler (into a curve) and bending as it is used here (changing from one straight-line path to another)

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Any change in medium

Wrong Track: Refraction is when light goes through glass blocks.

Right Lines: Light usually changes speed as it changes medium. Refraction of light is the change in direction of the beam and happens when light changes from travelling in one medium to another, if it does not travel in along the normal.

A sensible approach

Thinking about the learning

There is a tendency for pupils to associate refraction solely with what happens when light passes through rectangular glass blocks.

Thinking about the teaching

A sensible approach here is to introduce refraction by passing light through rectangular glass blocks and then to make explicit links to other phenomena which involve refraction.

Starting activity:

• Refraction through rectangular glass blocks.

Development activities:

• Refraction at air–water interfaces.
• Refraction by triangular glass prisms: Dispersion of light.
• Refraction by convex lenses.

A teaching anecdote

We're sure you'll find a story like this one to share with your class.

I was out walking with a friend when we came to a stream which was pretty wide (about 10 metre). It had been raining quite a bit in the previous few days and the stream looked full. I wasn't too sure about whether we could cross here. Perhaps we should look for an easier place farther down-stream. My friend would have nothing of it. It doesn't look too deep to me. I'm going to take my boots off, roll my trousers up and wade across. And so he did. In the very first step he lowered himself into the water from the bank, slipped and found himself almost waist deep in the stream.

I could feel an explanation about refraction, plus real and apparent depths coming on, but couldn't manage it through my laughter.

Teacher Tip: Work up a good story of your own about real and apparent depth, with illustrations.

There's a good chance you could improve your teaching if you were to:

Try these

• linking reflection to the act of seeing all non-luminous objects
• exploring the phenomenon of refraction through many different examples
• developing more complex examples on the foundations of an account of the simple phenomena
• making some real measurements of angles, emphasising skilful practice

Teacher Tip: Work through the Physics Narrative to find these lines of thinking worked out and then look in the Teaching Approaches for some examples of activities.

Avoid these

• restricting examples of reflection to only, or mostly, shiny surfaces
• replacing experiences of real and interesting phenomena with a series of ad-hoc memorised rules
• not showing how an understanding of simple situations is linked to a wide variety of phenomena in the lived-in world

Teacher Tip: These difficulties are distilled from: the research findings; the practice of well-connected teachers with expertise; issues intrinsic to representing the physics well.