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Seeing things - Teaching and learning issues
Reflection
Light, Sound and Waves
Seeing things - Teaching and learning issues
Teaching Guidance for 5-11
The Teaching and Learning Issues presented here explain the challenges faced in teaching a particular topic. The evidence for these challenges are based on: research carried out on the ways children think about the topic; analyses of thinking and learning research; research carried out into the teaching of the topics; and, good reflective practice.
The challenges are presented with suggested solutions. There are also teaching tips which seek to distil some of the accumulated wisdom.
Light travels
Wrong Track: When a light is switched on the light fills the room and gets everywhere. The light is just there.
Right Lines: Light is travelling in straight lines, in all directions, in any illuminated space. The light fills the room to the extent that it is travelling through it.
Emphasising the point
Thinking about the learning
Some pupils entering high school are likely to have little idea of light being something that travels in the space between its source (for example, a torch) and the effect it produces (for example, a patch of light on the wall). They think of light as being a source or an effect.
Other pupils think of light as existing in space, but will say that it is just there
. It does not move or do anything as such, but forms a pool of light
around a light source.
Here light is considered to be like a pleasant aroma that fills a room. This point of view is reinforced by some of our everyday ways of talking: At that moment the entire hall was bathed in yellow light.
Or the light from the security lamp filled the room when the intruder entered
.
Thinking about the teaching
There is a significant job for the teacher in working with those pupils who think of light only in terms of a source and effect. The question of where light comes from in the first place will need to be posed and ideas of light travelling from source to effect introduced.
It is an interesting shift away from common sense to consider that your living room is light
during the day because light is travelling through it in all directions and that it only looks light because light reflects off surfaces or any dust particles in the air and passes into your eye. At night when it is dark the only reason it is dark is that no light is travelling through the room. This is one of those cases where introducing a physics-based perspective necessarily involves making the familiar seem a bit odd.
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How do we see?
Common explanations
Wrong Track: The light is just there so that you can see things. If the light is on, you can see.
Wrong Track: You can see by looking at things. You just have to look at them.
Wrong Track: The light just helps your eye to see. It goes into your eye and then onto the thing you're looking at.
Right Lines: You are able to see as a result of light entering your eye. Light may come directly from a luminous source, or reflect off an object, or be scattered from particles and then enter the eye.
Pool of light
Thinking about the learning
For these pupils there is no sense of light travelling and entering the eye.
Active eye
Thinking about the learning
For these pupils seeing happens through light passing from the eye to the object. This model of seeing is reinforced by lots of expressions used in everyday talk. Thus we throw glances
, give someone a dirty look
, try to penetrate the fog with our eyes
. All such expressions suggest that our eyes are active in sending out something.
Light-to-eye-to-object
Thinking about the learning
For these pupils seeing happens as a process of light passing to the eye, and then from the eye to the object, and then back from object to eye.
Challenging the ideas
Thinking about the learning
You need to help pupils understand that we see things as a result of light entering our eyes. For many pupils at the start of high school, this is not a problem. Nevertheless, there will be a range of ideas represented in the pupils that you teach and these are likely to include the previous three wrong track
ideas.
Thinking about the teaching
How might you address some of these wrong track
ideas in your teaching?
As outlined earlier, the pool of light way of thinking is very close to common-sense ways of talking about events. For example, if we use a torch to look for a pair of trainers in the cupboard under the stairs, there is a strong sense of the torch lighting up the space
. Here, less thought is given to the torch sending out light, which reflects off objects and then enters the eye allowing us to see
.
The idea that seeing depends on light entering the eye is further explored through activities in the teaching approaches.
The active eye
model is best challenged through pupils experiencing complete darkness.
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How far can light travel?
Reasons for not seeing glowing objects
Wrong Track: The light runs out before it gets to us.
Wrong Track: Light's only got quite a long range: then it stops.
Right Lines: Light keeps on going until it gets absorbed. But there might not be enough light entering your eye for you to see something – although your dog might still be able to see it.
Light doesn't run out or get used up
Thinking about the learning
On the one hand pupils are quite prepared to accept that light can travel 150 million kilometres from the Sun to the Earth and yet at the same time believe that light from their torch beam gets used up
in a matter of a few metres.
The light from the torch becomes progressively more spread out and may also be scattered and absorbed by particles in the air (more in the physics narrative). The light is not lost
, it just becomes more spread out, scattered and absorbed.
This applies to the very bright sunlight, or to the somewhat dimmer torch. Light just does not get lost
.
Thinking about the teaching
Although the spreading and scattering of light are not referred to in all curricula, they are certainly necessary to offer any kind of plausible explanation for the relatively short range of a torch beam. One teaching colleague regales his pupils with the mantra:
Teacher: Light doesn't run out, it spreads out!
Furthermore, the only reason that you can actually see the torch beam cutting through the night air is because light is being scattered to your eye. These are ideas that pupils are ready to accept and we would advise that you use them. See the teaching activity that depends upon the scattering of the intense laser beam by chalk particles in the air.
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Reflection from any surface
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.