Light_Concept_1

Subject Facts

Light sources

Light isn't just ⁱaü around us'. It comes from somewhere and enables us to see things, There are only two means by which you can see something: either it a source of light or it reflects or scatters light produced by a source.

On a dull, cloudy day, it can sometimes be easy to forget that the Sun is our main source of light.The light from the Sun penetrates the translucent cloud cover to the ground below. Sunlight is also accompanied by heat radiation from the Sun, which is similar to visible light but is beyond our ability to see. When or where there no strong source of natural light (in the night or in a place that sunlight cannot reach), we make other light sources — usually by burning substances (such as gas or wood) or by passing electricity through them (fluorescent tubes, light bulbs and so on).

In addition to these, some interesting light sources come under the heading of bioluminescence: light from livings.

Examples include certain types of fungi, insects (such as fireflies), and deep-sea fish. These are able to glow by means of a biochemical reaction.

Some highly reflective objects may give the impression that they are luminous — but they are just able to focus and direct the light falling onto them effective:y. The rear reflectors on bicycles are good examples of this.

Light Travels in straight lines

Anyone who has read Stephen Hawking*s A Brief History of Time will know that tight curves when close to a black hole. But as the vast majority of us are unlikely to encounter this phenomenon, the 'straight line' Idea is one that we can trust for practical and teaching purposes. Light travels in a straight line from a source; when it changes direction, as it may do after making contact with an object, it always goes off in a straight line,

Mirror mazes give us a good idea of how difficult life would be if light not always travel in straight lines and so we couldn't rely on our eyes to judge the positions of things. In these mazes, mirrors are positioned to give false impressions — for example, that there is a long corridor ahead when, in reality, a mirror has been positioned to reflect the image of a corridor. Someone standing in the corridor would appear to be in front of you when he was actually standing around the corner. You would walk towards him and end up bumping into the mirror (see Figure l). if light travelled in curves, you would have similar problems: objects would appear to be in one place, but actually be in another The ability to see around corners might sound useful — but not if you can't be sure that what seems to be there is actually there!

Another way of proving that light travels in straight lines is to look at the shadow cast by a clear-edged object from a single, small light source. Such a shadow always has clear edges. If light did curve the edges of the shadow would be blurred because of light spilling around the edge of the object.

The nature of light

Where there isn't any light, there is darkness. Darkness isn't a tangible 'thing', it's just the absence of any light source to stimulate our eyes. This can be quite difficult to appreciate, because total darkness is quite difficult to achieve: even at night, there will be stars or street lamps to provide background light. For this reason, our society has become used to having to do something actively to keep Eight out: curtains, blinds and so on. The only time in my life that have been in total darkness was when I turned out my helmet tamp when I was in a deep pothole.

Nobody is exactly sure what light is. Sometimes it behaves like a form of wave energy, and at other times it behaves like subatomic particles (which are called 'photons'). Neither theory fully explains what light can do, so the two theories are used to explain different properties of light. This may be a bit confusing and unsatisfying, but scientific models are often not 'realistic': it is just a matter of using the model that works best to explain things.

We know that light (unlike sound) does not need anything to travel through. It is able to travel through outer space (since we can see the Sun and stars), which is mostly empty. This suggests that light is a particle rather than a wave, since waves could be said to need a medium to travel through. However; different colours of light are usually described as having different wavelengths, which implies a wave theory of light. More recently the colours of light have begun to be understood in terms of particles at different energy levels which are detected by our eyes as different colours.

Another key thing about light is that it travels at a particular speed called 'the speed of light'. Light is the fastest thing there is: nothing can travel faster (apart from spaceships in science fiction). Light travels at approximately 300,000,000 metres per second (see Amazing facts below). However; light only travels at that speed in space: travelling through transparent materials such as air; water or glass will slow it down to varying degrees.

Safety considerations

Very bright can damage eyesight by 'overloading' and in effect burning out the light-sensitive cells in our eyes. Sources of bright light may also be sources of heat (as in a light should therefore be handled with care.

Why you need to know these facts

For children to develop a correct understanding of other ideas about fight, they need to appreciate that light must come from somewhere, and that it always travels in straight lines,

Vocabulary

Darkness the absence of light.

Luminous able to produce light.

Amazing facts

The speed of light is 299,7921458 metres per second sprinter can run at 10ms .A car at 70mph is moving at 31ms. The speed of sound in air at sea level is 343ms. Concorde flew at 640ms . Light would travel almost seven and halftimes around the Earth in one second, if you could make it curve! It takes almost 8 minutes and ! 5 seconds for the Sun's light to reach the Earth.

The worlds largest solar power plant in Andalusia, Spain, uses 600,000 mirrors to focus the heat of the Sun onto a boiler. o A solar-powered car crossed Australia in 33 hours 1996 — an average speed of 85 kilometres per hour.

Common Misconceptions

The Moon is a light source.

Reflective objects in space at night can be confusing to us, because they appear to shine in darkness. Whereas stars (much tike the Sun, but further away) produce their own fight, the Moon and the planets (see Earth and Space) do not. We are only able to see those parts of the Moon that are illuminated by the Sun.

Light is all around us.

The idea that Eight simply hangs around rather than emanating from a source can persist into adulthood. The link between the Sun and the pattern of day and night is explored in more detail in Earth in Space, The need for a light source if there is to be light can be demonstrated in a blacked-out room with a torch. Challenge the children's understanding with questions to make them consider their ideas in greater depth: If light is all around us why isn't it under the bedclothes at night? Why do you need to turn the light on in the cellar? Sometimes it is difficult to make the link between light and its fight source: the Sun may be behind clouds, or it may be fight in a room when the Sun is not shining directly through the window. these cases, tight has been scattered into our eyes from objects above or around us.

Questions

Where does the light come from on a cloudy day when you can't see the Sun?

If the child has already grasped that light comes from a source and travels in a straight tine, he or she may weft wonder how light can reach you when you can't see its source directly. But even on a sunny day if you are standing in the shade and cannot see the Sun, it is still light. Firstly, the light can reach you by bouncing off other things (scattering or reflecting — usually scattering). Secondly, when the light source is obscured by something translucent (such as pale clouds), the light can still get through even though the image of the source is hidden. Frosted g!ass and paper lampshades are examples of this: because the material is translucent the fight is scattered (or diffused) through .

Teaching ideas

Sources of tight (exploring and sorting)

Provide a collection of artefacts: some light sources and some non luminous objects with a link to light (such as mirrors, bicycle reflectors and reflective strips). Children can examine and discuss these, then sort them into 'light sources' and ^E not light sources'. Some objects can be tested by placing them in a lightproof box which has a peephole cut into it.

NB Naked lights such as lit matches or candles, if used at all, must NOT be placed in a box. The colours of naked flames are very interesting to observe — but any such observation must be carefully supervised.

Light travels (exploring and explaining)

Children can explore a darkened room with torches. Try to get across the idea of a path from the Eight source to an object to the eye. Encourage them to explain where the light coming from, and where ft is going. They can record their understanding of how they see an object by drawing a picture with lines and arrows to show the path of the light.