Bringing together two sets of constraints

Focusing on the learners:

Distinguishing–eliciting–connecting. How to:

• identify source and detector
• separate luminous from non-luminous
• draw out children's ideas about seeing
• connect seeing to the source–medium–detector model
• build an explicit model of seeing

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:

• construct the source–medium–detector model
• show illumination as a process where something travels
• build a correct model of how illumination diminishes
• emphasise that there is a finite trip time from source to detector
• draw out the role of light in enabling seeing
• bring to mind the role of reflection in seeing luminous objects

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.

Objects, media, sources – all mean something special

Wrong Track: That object over there is shining, so we can see it.

Right Lines: Light comes from the source, and bounces off the object. We see the object when the reflected light enters our eye.

Light vocabulary

Thinking about the teaching

Teaching and learning about light involves using familiar words in specialist ways. For example: source means where light comes from; object means the thing that is being looked at; medium means the background through which light travels, whether it be a vacuum, air, water, or glass.

Using these terms allows you to talk in general ways about systems involving light. For example, it doesn't matter whether you are looking at a car, a book or an elephant, you can simply refer to it as the object.

Rather than just dropping these words into your teaching it is worth drawing pupils' attention to their usage.

Teacher Tip: Make the introduction and utility of technical terms an explicit part of the learning.

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.

Camp site shadows thrown by beams of light from the Sun

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

Recently I went on holiday to central Southern France. We were camping on a site in the Lot valley and on each side of the site there were high limestone cliffs. The weather was very hot during the day and towards late afternoon, the Sun gradually sank down behind the cliffs on the south side of the valley, creating a shadow. The fascinating thing about the shadow was the absolute sharpness of the line between light and dark, which crept across the camp site. On the dark side the Sun's beams were blocked out by the cliffs and on the other side the Sun's beams fell on the meadow. I couldn't help thinking that the light had travelled 150 million kilometres from the Sun, but the line between dark and light was clear cut, with no sign of blurring where the beams might have wobbled from their straight line paths as they travelled down from the edge of the cliff hundreds of metres above.

Teacher Tip: Work up a good story about shadows – perhaps with images to back it up.

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.

Cats and blackout

Angela, a year 7 pupil, came out of the darkroom and was asked whether she was able to see her hand in front of her nose. No, replied Angela, but I bet our cat could have done!

A true story, and worth raising as a further discussion point. Can cats see in the dark? Why do you think that Angela believes her cat can see in the dark? What is it about cats' eyes?

Warning: One year after a blackout activity two boys asked whether they could try to make the science lab a light-tight enclosure. For two weeks they came up at lunch-times and worked with rolls of black paper, blanking off the gaps around windows and doors. Eventually they gave up, admitting that it was impossible to stop all of the light. The technician breathed a sigh of relief!

Teacher Tip: A good room black-out is essential for this and all of the other activities relating to light. On first consideration this particular demonstration may seem to be lacking in potential impact. Experience, however, has shown that the combination of full black-out and a powerful point source of light leads to a quite striking demonstration.

Sight and light

If there is no light available to reflect from the surface of an object, then you will not be able to detect (see) it. The simple, and pretty obvious, rule is:

Teacher Tip: No light: no sight.

Can you say how the light is provided to aid the sight for items in these three images?

Light dissipating

The fact that we can see the Sun and stars shows that light can travel over enormous distances (150 million kilometres from the Sun). In fact there is no known limit to how far light can travel. However, as you will be aware from observing torch beams or car headlights, there is a limit to the distance over which these are effective sources of illumination.

There are two reasons for this.

First, as light spreads from a bulb the level of illumination it provides is reduced (this is why a given source of light appears dimmer as you get further away from it).

Second, some of the light from the bulb will be scattered and absorbed by particles in the air and this further reduces the level of illumination it provides at any given distance.

Why is it safe to look at the red laser spot on the wall and not at the laser directly?

We'd suggest that it pays to emphasise that laser light is reflected from the rough surface of the wall in all directions (diffuse reflection), so that all of the class can see the spot no matter where they are sitting. For each observer only a very small fraction of the original beam passes into their eyes, which means that it is safe.

Light does not just get here in no time at all

Wrong Track: Light gets there straight away: there's no gap between the lightning and the place being lit up.

Right Lines: Light does travel very quickly: but it still takes time to make the journey from the source. The speed of light is large, not infinite.

The speed of light

Thinking about the teaching

The speed of light, in metres per second, is one of those huge numbers that is occasionally thrown up in science and that can capture pupils' interest and imagination. A good starting point to help pupils appreciate the magnitude of this speed is to consider speeds that the pupils will be able to imagine:

Teacher: Does anybody know what the world record is for the 100 metres?

Linford: It's 9.58 second!

Teacher: Spot on! Let's say about 10 second. The fastest person can run 100 metres in about 10 seconds.

Teacher: How far does light travel in just 1 second?

Teacher: It's more than 100 metres.

Teacher: It's more than 300 metres.

Teacher: It's about 300 million metres in each second!

Another way to approach this, for more mathematically able pupils:

Light travels 300 million metres (3 × 10⁸ metre) in 1 second; or 300 thousand metres (3 × 10⁵ metre) in 1 millisecond; or 30 kilometres (3 × 10⁴ metre) in 0.1 millisecond.

In other words, a beam of light will travel from Newcastle to Sunderland (insert local reference) in one tenth of a thousandth of a second!

The main point here is that whilst the speed of light is extremely big it is also finite. Light does, therefore, take time to cover very large distances. For example, when you see the light from very remote stars, you are actually seeing light emitted from them a long time ago. You literally see them as they were in the distant past. Closer to home, you see the world 30 cm away from you as it was one nanosecond ago, not as it is now.

Links to the speed of light, including light years

Pupils are invariably fascinated by the links which can be made from the speed of light to distances across space, and travel times through space. It is really helpful to have some relevant facts and figures at the ready.

The light year is used in astronomy as a unit of distance that helps to deal with the vast scale of the universe.

1 light year is the distance travelled by light (through a vacuum) in one year and light travels 300 million metres in one second, so light travels (300 million)  ×  (365  ×  24  ×  60  ×  60) metre in one year.

Therefore 1 light year is 9.5 × 10¹⁵ metre.

For example, Supernova 1987a (a supernova is the explosion at the end of a massive star's life-time) occurred in a nearby galaxy called the Large Magellanic Cloud. Light from this supernova was observed on Earth in 1987, but the distance to the Large Magellanic Cloud is about 190 000 light years. Thus, we normally say that Supernova 1987a occurred in 1987, but it really happened about 190 000 years earlier. Only in 1987 did the light of the explosion reach the Earth! If we want to know what the Large Magellanic Cloud looks like now, we will have to wait 190 000 years.

In comparison, the Sun is only about 8 light-minutes away. That is, it takes 8 minutes for light to travel from the Sun to the Earth. So the light we see from the Sun represents what the Sun looked like 8 minutes ago, and we must wait another 8 minutes to see what it looks like now. It is an interesting fact that if the Sun went out we would not know about it until 8 minutes after the event! Reflected light from the Moon travels to the Earth in about 1 second.

• The nearest star to the Earth (other than the Sun) is 4.3 light years away.
• Our galaxy (the Milky Way) is about 100 000 light years in diameter.
• The distance to the galaxy M87 in the Virgo cluster is 50 million light years.
• The distance to the most distant object seen in the universe is about 12 billion light years (12 × 10⁹ light year).

The most distant things that astronomers can see are about 12 000 000 000 light years away. Thus, the light that we presently see from these objects began its journey to us about 12 billion years ago. Since that is close to the estimated age of the universe, this light is a kind of fossil record of the universe not long after its birth! Thus the observation of very distant objects is in a very real sense equivalent to looking backwards in time.

And it works at all distances, even within the classroom. History is rushing in at you as fast as it can, 30 centimetre every nanosecond (0.000 000 001 second)!

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

Try these

• exemplifying seeing in the lived-in world using the source–medium–detector model
• emphasising the trip time as a delay between source and detector
• working up some convincing demonstrations that light seems to travel in straight lines
• always explicitly accounting for reductions in brightness

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

• speaking or acting as if light was just there
• assuming that how we see is well understood
• assuming that seeing is all accounted for by the physics
• conflating rays (the theoretical construct) with light beams (the physical)

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.