BuiltWithNOF
Light

Some materials allow light to travel through them and others don’t.

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Draw some crosses of different sizes on a piece of paper.

Pick out the three different types of plastic in the picture.

Try placing each piece of plastic in turn on top of the crosses and describe what you see.

 

The red piece of plastic is described as opaque.
This means it does not let any light travel through it.
We cannot see the crosses when we place it on top of them.

The clear piece of plastic is described as transparent.
This means it will let light travel straight through it.
We can see the crosses clearly when we place it on top of them.

The whitish piece of plastic is described as translucent.
This means that only some light can pass through it. The light is also ‘spreads out’ as it passes through this material and this has the effect of blurring the image we see.
We can see the crosses but not as clearly as with the transparent plastic. Try lifting the material up from the text or crosses and you will notice that they become impossible to see clearly.

Materials can be various colours. A coloured material could be either transparent, translucent or opaque. Look out for examples like this. Shop signs and window arrangements often have combinations of these materials with lights behind them to make them stand out or so that names can be clearly seen and attract attention.

Materials which are transparent and curved can alter the light travelling through them in a special way. They can bend the light. Many useful inventions have been made because of this fact.

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Pick out the rod (cylinder) of transparent plastic.

Place it on the crosses and slowly roll it back and forwards. Look at the crosses through the plastic rod and describe what you see.

 

Try lifting the rod up off the paper a little and bringing it nearer and further away.

Try holding the rod up to your eyes – carefully – and looking at different objects in the room.

Try looking at your partner and slowly blinking. What does your partner see?

The crosses change shape and get bigger. The curved transparent plastic allows light to travel through it but it also changes the light coming from the things we look at. The rod can magnify writing and it can make it look upside down but this depends on how near you are to it.

If light meets a shiny material it bounces off. We can see reflections in shiny surfaces and we can see our faces in a mirror.

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Try holding the mirror on an edge and bringing it near the crosses. What can you see?

Move the mirror nearer until the edge is along the centre of the crosses. What can you see?

Try drawing other shapes such as squares, triangles and circles and placing the mirror on top of them.

 

Try drawing half a face and watch the rest of the face appear in the mirror.

When the face is finished try moving the mirror nearer or further away and try tilting the mirror to see what happens to the face. This mirror has a piece of blu-tak behind it to keep it in place.
 

Try writing a word and looking into the mirror. The letters are completely reversed.
Have you ever noticed the writing on the front of a
FIRE engine?
Try deliberately writing a word in reverse then see if you can read the word using the mirror. When we look into a mirror we see a reflection of ourselves which is reversed. You can prove this by standing in front of a large mirror and holding up your right hand or by looking into a small mirror and blinking one eye. Which hand is being held up in the reflection and which eye is blinking back at you? It takes a little bit of thought to work this out! If mirrors did not reverse the image we see then this would be very confusing - why? Think about combing your hair in a mirror for example.

If one mirror can make one reflection then two mirrors can make..............?

In fact two mirrors can make lots more than just two reflected images and you can prove this by standing two mirrors edge to edge at an angle to one another using some blu-tak at the back. Now bring a pencil point close to the corner and count the number of pencil images you see.

 Now try moving one mirror to make the angle smaller.
What do you see?

The light from the pencil (the ‘object’) is being reflected many times by the mirror and creates lots of reflected ‘images’. You can see this effect at a fairground in the ‘hall of mirrors’.

Light is a form of energy which travels in straight lines and spreads out from a source such as a candle, torch or the Sun.

In order to find out more about light we need to make a thin beam or ‘ray’ of light. A torch usually makes a wide beam which is good for guiding us in a dark place but not very good for light investigations. There are too many light rays coming from the torch and they are not all traveling in the same direction!

A simple way to make a thin ray of light is to make a thin slit in some card and then shine the torch onto the slit. A thin but bright ray of light emerges from the other side.

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In this experiment the torch is in the bottom right hand corner and is shining onto the card slit. A thin ray of light can be seen traveling out from the slit.

A mirror held up by some blu-tak is placed in the path of the light ray from the slit. The light ray reflects from the mirror and changes direction. The angle at which the light ray strikes the mirror is the same angle at which it reflects off from the mirror. This can be proven by drawing pencil lines along the light rays and then measuring the angles using a protractor.

If two mirrors are used the light ray is made to change direction twice.

In this picture the torch is at the left hand side. The slit is near the middle of the picture and the single ray of light can be seen traveling right where it reflects from the mirror and travels downwards to the second mirror. It changes direction again when it strikes the second mirror and travels to the right again.

This double reflection is made use of in a very simple but important invention called the periscope.

This picture shows a periscope in use in the trenches in the Great War of 1914 to 1918.

The soldier looks into a mirror in the bottom of the long narrow box and sees the light coming from over the top of the trench wall. There is a mirror in the top of the box collecting light from the battlefield and reflecting it down to the mirror in the bottom.

It allows the soldier to see what is happening on the battlefield without having to put his head up over the trench wall where the enemy might be able to shoot him!

The diagram below is called a ‘ray diagram’ and shows how the light rays from objects on the battlefield travel through the periscope to the eyes of the soldier who is safely down in the bottom of the trench.

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Transparent, Translucent and Opaque materials are all around us and each has special uses. When these materials are used together we can produce some amazing gadgets and inventions which we couldn’t do without.

Did you know?

The Chinese invented the ‘cinema’ and projected onto screens thousands of years ago!

Perhaps calling it a cinema is stretching it a bit but they used candles and shiny reflective metal surfaces to direct a light beam onto a paper screen and placed cut-out puppets and masks in between to cast shadows onto the screen. The effect in a dark room was amazing and they made up all sorts of adventures and told ancient stories in their shadow puppet theatres.

The screen was translucent and this meant that the audience could see the shadows but not the people moving the puppets nor the light source. Screens like this are still used in some cinemas and for special effects - we call it ‘back projection’ because the light comes from behind the screen not in front as in most projection systems.

The puppets were made of opaque materials eg. thick card or wood.

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It’s really simple to make a shadow puppet theatre!
You will need a torch as the light source and some card as the opaque material with which to make a shadow puppet eg. a face with cut-out eyes etc.
The screen is a translucent material such as tracing paper or the whitish plastic from a plastic milk bottle.
You can use a transparent material to help obtain coloured eyes or features in the shadow produced on the translucent screen - coloured cellophane sweety papers are great for this!

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A thin plastic drinking straw or cocktail stick is used to hold the puppet so the your fingers don’t get in the way and cast a big shadow.

You will have to line up the torch, puppet and screen in order to cast the shadow onto the screen in the right place.

Remember to view your shadow show from the opposite side to the puppets and torch!

What sort of puppets and stage props will you design?

What story will you perform?