Light: From bonfires to lasers Supports National Curriculum, Key Stages 1D, 3F, 6f suitable for years 4,5 and 6



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Light:

From bonfires to lasers
Supports National Curriculum, Key Stages 1D, 3F, 6F
Suitable for years 4,5 and 6.
The time for whole session is about 1 hour 15mins. This can be varied by taking shorter paths through the material eg by omitting activities in italics or omitting the section on colour. Choices will depend on the particular needs of the school.
Outline of content

Aims to

  • establish what the children already know, using demos and class experiments

  • explain that excited atoms give out light in cold objects as well as hot objects

  • understand that we see when light travels to our eyes

  • understand that darkness is absence of light and we cannot see in true darkness

  • find that we see most objects because they reflect light to our eyes

  • find that mirrors change the direction in which light is travelling and that the angle matters

  • show white light is made up of colours

  • explain primary colours of light are red, blue and green

  • investigate coloured materials in different colours of light

  • demonstrate and discuss up-to-date applications by linking the children’s knowledge with their experience.


Points to note:

Please read the notes about risks and agree the assessment with the teacher before the session!

Cross References

- The slides in the PowerPoint presentation are referenced in the table.

- As the presentation may be running throughout the session some simple slides provide a suitable background, alternatively switch to a black screen.



  • Apparatus details are below the table and are linked to the relevant sections.

  • Notes about safety are below the table

  • Vocabulary: The presentation uses expressions included in the KS1&2 strategies http://www.standards.dfes.gov.uk/schemes2/science/?view=get

  • NB The phrase ‘excited atoms’ does not occur in these strategies.


Misconceptions – ie - Points that are often misunderstood

    • Only hot objects give out light.

    • We see by light leaving our eyes as shown in films about robots.

    • Blackness is a form of light rather than an absence of light.

    • We can see in the dark.

The session should be held in a room that is not brightly lit. Curtains or slat blinds could be shut but the activities have been planned so that blackout is NOT required as this is difficult to achieve in most primary schools and also might cause behaviour problems.


Light: from bonfires to lasers






Activities

Discussion

Aims/facts

1

Sources

Snap a glow stick so that it glows brightly. Ask what is happening.

Hazard: see safety notes.

See apparatus list.



Encourage answers such as the stick is giving out light. Several sticks might be handed round then placed prominently to be seen throughout the session.

Objects that give out light are called sources of light.

2

Suggest that they are camping and it is getting dark. How can they provide some light?
Slides 2 to 5

(Bonfire, sunset, stars and lightening)




Torches and a fire will be suggested. Slide 2

Suggest they imagine going for a walk in the dark on a moonless night without torches or matches. Will there be any light? (stars)



Hot objects emit (send out) light.

Light travels from a source.


3

Discuss other natural sources of light.
Slide 6

If it is very dark, when camping, they might be lucky to see a firefly, luminous fungi or a glow worm.

Sources of light vary in brightness


4

Show a toy which uses LEDs.

See apparatus list.

Ask for volunteers to handle the toy after it has been on for a few minutes. They should notice that it is still cold.

Other cold sources of light:

TV

LEDs


Light sticks -Chemical reaction

Phosphorescent plastic

Show slide 7


Discuss that not all sources of light are hot.

Point out that the light sticks are still cold after giving out light for several minutes.


(Remember that the moon is not a source of light.)

Some sources of light are cold.

5

What is happening?

Explain that everything is made up of tiny particles called atoms. After atoms are heated or have been excited in some other way, they give out light.

Light is given out by atoms.

6

Phosphorescent paper or plastic

It is possible for some materials to take in light then give it out later.




7

How do we see a source of light? Light goes from a source to our eyes. Our eyes then send messages to our brains that enable us to interpret the pictures detected by our eyes.
Slide 8

Discuss that if they look at the room lights light travels directly to their eyes.
(If they ask how we see objects that are not sources, then explain that we will come to that soon but do not detour at this point.)

We see sources of light as the light can travel directly to our eyes.


8

Ask whether we can see anything if we are in a completely blackened room?

Take a vote on this.

Slide 9


Then discuss waking in the night in the dark. Those who think they can see in a completely dark room will recognise that there is usually a small amount of light (eg through the curtains) and once our eyes are used to the dark we can see when there is very little light.

Darkness as the absence of light.
We need light to travel to our eyes to be able to see.

9

Hand out the black bags – one to each group.

They should put a few small objects into the bags.

The children should look into their bag, one at a time holding the bag close to their faces.

Then they should shine a torch into bag so the objects can be seen.

See apparatus list.


Discuss that when there is no light in the bag we cannot see inside it.

Then when the torch shines in the bag they can see because light bounces off the objects in the bag and travels to their eyes.

Slide 10


We can see objects that do not give out light when light shines on them and bounces back to our eyes.

10

Switch off the torch and they should see the stickers glowing brightly but other non luminous objects will be difficult to see.

Encourage them to ask about the stickers and explain that they have taken in some light and can send it out later because they are phosphorescent.

Phosphorescent materials absorb the light and send it out later.

11

Slide 11

Ask if they think the baby squirrel is keeping watch as it feeds.

Discuss that the squirrel will be watching who is near because its eyes are wide open. It will be able to see people because light bounces off them, and travels to its eyes.




12

Reflection Slide 12







13

Slide 13

Allow light from the slit slide to shine on a non reflective surface e.g. a white board or sheet of pale cloth. The surface will look brighter because some light bounces back to our eyes.



The light is scattered in many directions like in slide 10.


When light bounces off something we say it is reflected.

14

Hold a large mirror in the beam of light from the slide. Change the position of the mirror until a bright line is seen on the wall.

Light reflected as a beam.

Slide 14

See apparatus list.

See safety notes.


Explain that the light is not scattered so the beam can be seen when it reaches a surface. Shiny surfaces can be used as mirrors, dull surfaces cannot.

Shiny surfaces reflect light better than dull surfaces. Dull surfaces scatter the light.
We represent the light ‘beam’ as an arrow.

15

Change the position of the mirror so that the line on the wall moves.

Slide 15



A child could be asked to hold a second mirror to reflect the beam again. Discuss that the direction that the light is reflected depends on the position of the mirror.

Point out that we can predict where the light will reach the wall because light travels in straight lines.




Mirrors can change the direction of the light.


16

Game

A large mirror is fixed or held at the front of the class and children take it in turns to point a torch at it so that the light reaches a particular object in the room.


See apparatus list.

The child holding the torch finds that the torch should be pointed at the image of the object in the mirror. With a bright torch the game is possible in a room that is well lit but not flooded with direct sunlight.




17

Slides 16 and 17

Hand out a mirror and a torch to each group. Ask them to shine the torch towards the mirror and look at the reflection and see that the reflection moves as the torch moves. See safety notes.

Then each group should prop a mirror upright on a sheet of white paper. Then lay a torch on the paper pointing towards the mirror. A beam of reflected light should be seen on the paper. If a child looks into the mirror the reflection of the torch is seen.

See apparatus list.

(Slide 18 is an optional extra.)


They will be able to see that if the angle at which the light meets the mirror is changed then the direction of the reflected ray changes.

Discuss that when the light meets the mirror at a small angle it is reflected at a small angle etc.



When the angle at which the light hits the mirror is changed, the path of the reflected light also alters.

(At this stage children do not need to know that the angle of incidence is equal to the angle of reflection.)

18

Shadows

These are covered in the presentation on Sunlight.

However, if objects are held in the beam from a data projector then they can be explained.

This should only be covered if particularly requested. Points to cover are the factors which might affect the size and position of the shadow.




19

Colour

(Use slides 19 and 20 as introduction.) Each group should shine a torch on a CD to see the reflected colours.

Slide 21
See apparatus list.


Discuss the fact that the light falling on the CDs is white light (not coloured) and ask where the colours can come from. Explain that white light is made of colours.

White light can be split up into colours.

20

Slide 22, 23

Show the slide of the rainbow.





Encourage the children to compare these effects with the CDs and explain that the water droplets in the air split the light into colours.




21

Hold a large plastic sheet of diffraction grating in the beam from the data projector. Bright colours will be seen and yet when it is moved from the beam the colours vanish.

Slide 24 See apparatus list.






White light is made from all the colours of the rainbow.

22

Hand out a small piece of grating per child or per group and switch on a small clear electric lamp bulb.

The children should look at the lamp through the grating.



The children will see a bright pattern of colours - the colours are in the same order as in a rainbow.




23

Colour disc

Spin the colour disc holding it high so the whole class can see. If possible increase the speed slowly until the disc appears white. See apparatus list.

See safety notes.


Discuss that the colours add to give white. They might like to discuss persistence of vision to explain the effect.

The colours of the rainbow add to give white light.

24

Ask if they mix paints. Explain that there are primary colours in light (red, green, blue) which are different to those in paints (red, yellow, blue) and this leads to some odd effects.

Slides 25, 26, 27 emphasise the primary colours and can be used later. (These colours are called primary because by adding these lights one can make all colours we see.)



Discuss mixing paint colours - encourage

red + blue = purple

green + blue = turquoise but they will also give

blue + yellow = green* and others that are less useful.




Primary colours of light are red, blue, green.


25

Show slides 29 and 30. They are as the children expect from paints.

Then show slide 31 They find the yellow surprising.




Explain that the colours overlap on the slide to give the effect. (Some children are familiar with PowerPoint and word.)

The words magenta and cyan might be introduced.

red+blue=magenta

blue+green=cyan

red+green=yellow


26

Use slide 32 which provides a beam of white light and hold up a coloured transparent film or other transparent objects in the beam.
See apparatus list.

Discuss opaque and translucent materials and therefore the idea of some light being unable to pass through some materials.

Translucent materials allow some light to pass through. (Not all translucent materials are coloured.)

27

Hold up coloured objects in coloured light using slides 25,26,27, 33,34,35 to show that objects look different in coloured light. See apparatus list

Choose good effects and cover this quickly without going into detail.

Choose objects such as a brightly coloured silk scarves. The IoP red and yellow women’s scarf is very effective for this. Explain that only some of the light colours can be reflected. The light that is not reflected is absorbed.




27

Hold up a dull black object in coloured beams of light and it will still look black because it takes in all the colours of light.




Black objects absorb all colours.

29

Slide 36

Wear a rubber glove to hold up a string of lit Christmas tree lights bunched together. Then hold transparent sheets of coloured plastic or silk in front of the lights. The appearance of the lights changes.

See apparatus list.

See safety notes.



Explain that some colours are absorbed and some can pass through. Eg only green light can pass through green plastic. Red, blue and magenta can pass through magenta plastic. The activity should be covered quickly but gives the opportunity to reinforce ideas.




30

Slide 37

Explain that research about light has led to the development of lasers, which have many uses. Eg CD players.



Remind them that earlier they had learned that light is given out by atoms. In a laser all the atoms can give out light at the same time so it is very bright.

Scientists keep finding out new exciting facts.

31

Slide 38

Explain that in a CD player, the laser light is reflected by the smooth sections and scattered by the pits to give a message, which is changed to an electrical signal and then music.



Discuss that the CD is circular because the smooth parts and pits are in a spiral pattern.

Learning about physics leads to useful inventions.

32

Slide 39





Physics is in everything.

Apparatus

Please read the safety notes below this apparatus list.

1

Glow sticks or glow bracelets etc can be bought cheaply on the web eg glow glasses for 39p each – cheaper if more than 10 bought Hazard- see the safety notes!


4

Bright toy using LEDs or a miniature torch using a LED.



9

Black bags – enough for one per group of children

Black cloth bags made from dense black material, for example from an old black tee shirt.

Phosphorescent stickers – available from toy shops and stationers

Torch – one per group. The school could be asked to provide these or the children asked to bring them.

The bags should be approximately 20 cm x 20 cm or larger. Stick a selection of phosphorescent stickers at the bottom of the bag the day before the session and then keep the bags in the dark.

When the bags have been given out to the groups, ask the children to put a few small objects into the bag (eg a pencil sharpener). Check with the teacher before the session that suitable objects will be available or put some objects in the bags before the session.

When they look inside the bag, (one child at a time) they will hardly be able to see the objects.

Then they should shine the torch into the bag so that they can see all of the objects.

After the torch is switched off they will see the stickers glowing brightly.
14, 15, 16

Mirror

Demonstration

A large mirror to reflect light from the PPT slide – at least 50 cmx50 cm- use a household mirror

A second large mirror – preferably plastic
17

Class experiment

For each group:


  • Torch

  • Mirror (Schools usually have mirrors which are also used in maths.)

  • Block or support to keep mirror upright

  • A sheet of white paper


19

Several CDs to hand round for the children to see the colours in the reflected light.



21

Diffraction grating material in the form of plastic film can be obtained from Edmunds Optics (USA) two sheets ~ 15x30 cm2, 1000 lines per mm for ~ £8 plus postage. Keep one sheet whole for demonstrations and cut the second into pieces for the children to use. (If the small pieces are framed with card they will not be lost.)



23

Colour disc demonstration

Copy the colour wheel below, print it and glue the disc to a circle of card.



(To enlarge the wheel, left click on it and stretch the corner dots that appear.)

I used a hand held food mixer with only one beater attached and fastened the card to it using a large lump of blu tac.(String can also be tied through small holes in the card to fasten it to the beaters or fan to ensure that the disc does not fly off at high speed.) It was possible to increase the speed slowly so that the children could see the colours blending more and more until at the highest speed the disc appeared white.

Alternatively attach the disc to the end of a travel fan or the bit of an electric drill.

26

Samples of coloured transparent plastic – I found that brightly coloured transparent wallets for A4 papers were suitable and much cheaper than colour filters. I chose the brightest colours and used the wallets fastened.


27

Collect a few large brightly coloured objects that you have available - I used posters and silk scarves. Use the slides to show that they objects look different in different colours of light.

Eg: a yellow object in red light then green light.

a blue object in red light

etc
Note - an explanation of why yellow and blue paints mix to give green

(this works for only some blues)

- yellow paint reflects red + green light

- some blue colours are actually blue + some green

- mixing paints is colour subtraction

- the reflected colour is the colour that can be reflected by all the paint

- hence green is seen.
29

A string of coloured Christmas-tree lights

(You will need to check that there is a convenient mains socket before the presentation.)

Samples of coloured transparent plastic as in 26


_______________________________________________________

Safety Notes
Do not use laser pointers as some of them are not safe.

It is dangerous for the children (or anyone else) to look at the Sun.
Light sticks:

If the outer casing of a light stick is punctured the fluid is hazardous because it:

  • may contain tiny shards of glass

  • should not be in contact with eyes

  • should not be ingested

  • can cause irritation

  • may cause an allergic reaction

  • may stain clothing, skin etc and should be washed off with soap at once.

Light sticks cannot be reactivated once the chemical reaction has ceased and

should be disposed of so that small children cannot have access to them.
Mirrors

  • Make sure that mirrors do not have sharp edges.

  • When using the large mirror hold it personally and do not ask the children to help.

  • Use plastic mirrors if possible for group work.

  • Do not reflect sunlight with mirrors. If sunlight is reflected into a child’s eye it can cause damage.


Fasten the colour disc to the mixer or fan securely so that it does not fly off.
The Christmas tree lights are connected to the mains electricity supply and so should not really be handled by anyone while powered. To make this safe (and to emphasise the risk) the hand holding the lights should be protected by wearing a rubber washing-up glove.


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