Experiments

Light: From bonfires to lasers

1. Sources

Aims/Facts

  • Objects that give out light are called sources of light.
ActivityDiscussion
Snap a glow stick so that it glows brightly. Ask what is happening.

View safety notes

View 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.

 


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

Aims/Facts

  • Hot objects emit (send out) light. Light travels from a source.
ActivityDiscussion
Show 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).

 


3. Discuss other natural sources of light. Show slide 6.

Aims/Facts

  • Sources of light vary in brightness.
ActivityDiscussion
 If it is very dark, when camping, they might be lucky to see a firefly, luminous fungi or a glow worm.

 


4. Show a toy which uses LEDs.

Aims/Facts

  • Some sources of light are cold.
ActivityDiscussion
View 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.)

 


5. What is happening?

Aims/Facts

  • Light is given out by atoms.
ActivityDiscussion
 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.

 


6. Phosphorescent paper or plastic

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

 


7. How do we see a source of light?

Aims/Facts

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

Show 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.)

 


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

Aims/Facts

  • Darkness as the absence of light.
  • We need light to travel to our eyes to be able to see.
ActivityDiscussion
Take a vote on this.

Show 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 (e.g. through the curtains) and once our eyes are used to the dark we can see when there is very little light.

 


9. Hand out the black bags – one to each group.

Aims/Facts

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

View 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.

Show Slide 10.

 


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

Aims/Facts

  • Phosphorescent materials absorb the light and send it out later.
ActivityDiscussion
 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.

 


11. Show Slide 11.

ActivityDiscussion
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

ActivityDiscussion
Show Slide 12. 

 


13.Show Slide 13.

Aims/Facts

  • When light bounces off something we say it is reflected.
ActivityDiscussion
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.

 


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.

Aims/Facts

  • Shiny surfaces reflect light better than dull surfaces. Dull surfaces scatter the light.
  • We represent the light ‘beam’ as an arrow.
ActivityDiscussion
Light reflected as a beam.

Show Slide 14.

View Apparatus List

View 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.

 


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

Aims/Facts

  • Mirrors can change the direction of the light.
ActivityDiscussion
Show 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.

 


16. Game

ActivityDiscussion
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.

View 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. Show Slides 16 and 17.

Aims/Facts

  • 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.)
ActivityDiscussion
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.

View 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.

 


18. Shadows

ActivityDiscussion
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

Aims/Facts

  • White light can be split up into colours.
ActivityDiscussion
(Use Slides 19 and 20 as introduction.)

Each group should shine a torch on a CD to see the reflected colours.

Show Slide 21.

View 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.

 


20. Show slides 22 and 23.

ActivityDiscussion
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.

Aims/Facts

  • White light is made from all the colours of the rainbow.
ActivityDiscussion
Show Slide 24.

View Apparatus List
 

 


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

ActivityDiscussion
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

Aims/Facts

  • The colours of the rainbow add to give white light.
ActivityDiscussion
Spin the colour disc holding it high so the whole class can see. If possible increase the speed slowly until the disc appears white.

View Apparatus List

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

 


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.

Aims/Facts

  • Primary colours of light are red, blue, green.
ActivityDiscussion
Slides 25, 26 and 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.

 


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

Aims/Facts

  • red + blue = magenta
  • blue + green = cyan
  • red + green = yellow
ActivityDiscussion
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.

 


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.

Aims/Facts

  • Translucent materials allow some light to pass through.
  • (Not all translucent materials are coloured.)
ActivityDiscussion
View Apparatus ListDiscuss opaque and translucent materials and therefore the idea of some light being unable to pass through some materials.

 


27. Hold up coloured objects in coloured light using Slides 25, 26, 27, 33, 34 and 35 to show that objects look different in coloured light.

ActivityDiscussion
View 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.

 


28. 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.

Aims/Facts

  • Black objects absorb all colours.

 


29. Show Slide 36.

ActivityDiscussion
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.

View Apparatus List

View 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. Show Slide 37.

Aims/Facts

  • Scientists keep finding out new exciting facts.
ActivityDiscussion
Explain that research about light has led to the development of lasers, which have many uses e.g. 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.

 


31. Show Slide 38.

Aims/Facts

  • Learning about physics leads to useful inventions.
ActivityDiscussion
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.

 


32.Show Slide 39.

Aims/Facts

  • Physics is in everything.