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Page 1: 8 k light (boardworks)

© Boardworks Ltd 20041 of 20 © Boardworks Ltd 20051 of 84

KS3 Physics

8K Light

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8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour

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Light carries energy and travels as a wave.

Light travels much faster than sound at a speed of 300,000,000 m/s, which is the same as 300,000 km/s.

Light waves travel in straight lines.

What is light?

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Opaque materials do not allow light to pass through them – transparent materials do. A material that is translucent only lets part of the light through.

Which materials let light through?

Hold different materials between the lamp and the screen.

Use the results table and shading chart on the next slide to estimate the opacity of different materials.

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Material Opacity (%)

Tracing paper

1 ply tissue

2 ply tissue

Smoked glass

Glass

Perspex

Wood

Opaque materials do not allow light to pass through them – transparent materials do. A material that is translucent only lets part of the light through.

Shading chart

75 %

100%

0%

15 %

2.5 %

50 %

30 %10 %

25 %20 %

5 %

Which materials let light through?

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1. Fill a clear glass trough or empty fish tank with smoke.

How does light travel?

Light waves travel in straight lines.

2. Use a slit to shine rays of light through the tank and describe what you see.

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Light from the object enters your eye.

Do you see all objects in the same way?

There are two ways you see objects:

You see some objects because they are light sources.

You see other objects by reflected light.

Seeing light

How do you see an object?

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A luminous object gives out light and can also be called a light source.

Light travels in a straight line directly into your eye.

Seeing a luminous object

How does light from a light bulb and other light sources reach your eye?

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Objects that do not give out light are non-luminous.

Light from the light source strikes the book and some of the light is reflected into your eye.

Seeing a non-luminous object

How does your eye see non-luminous objects such as a book?

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8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour

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Objects that do not reflect light well:

Good and bad reflective materials

Objects that reflect light well:

have smooth, shiny surfaces and are usually pale colours;

give clear images because they reflect light regularly;

mirrors are excellent reflectors.

have rough, matt surfaces and are usually dark colours.

give no or diffuse images because they reflect the light irregularly.

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Arrange these items along the arrow:

white paper

red

roses

polished black shoes

aluminium

foil

yellow

banana

blue car

tree bark

tangerine

green leaf

tarmac road

best reflectors

worst reflectors

Good and bad reflective materials

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What happens to light that is not reflected?

Light that is not reflected

Some of this light may be absorbed,

e.g. as heat.

Some of this may also be transmitted,

e.g. glass reflects a small amount of light, absorbs some of the rest and allows most of it to pass through.

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The following activities are designed to investigate the main laws of reflection.

Summarize each investigation with a law based on the results of the exercise.

Reflection investigations

1. Reading in mirrors.

2. How far away is the image?

3. The maths of reflection.

4. Reflecting without mirrors.

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In small groups, take it in turns to read the list of words on the next slide with your back to the screen using a mirror.

You can only move on to the next word when you have read the first word correctly.

Name Time taken to read

(s)

Natasha 46

Pashmina 56

David 85

Reading in mirrors – instructions

Put your results in a table like this:

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dog man ball

bat bike ants

park fins pink

litter sandy shark

Reading in mirrors – words

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1. Who read the words in the quickest time?

A graph showing the results of 'Reading in Mirrors'

0

50

100

Natasha Pashmina David

Name

Tim

e ta

ken

[s

]

Reading in mirrors – results

2. Plot a bar chart of your results:

3. What was the average time taken in your group?

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A plane mirror reflects light regularly so that it produces a clear image which is the same size as the object.

When something is reflected in a plane mirror, left becomes right and right becomes left. This is called lateral inversion.

Lateral inversion

What is different about the image?

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1. Fix a plane mirror along the centre of a piece of A4 paper and draw around it.

Place a pin as the object in front of the mirror.

2. Line up a ruler with the image of the pin and draw along the edge of the ruler on the paper. Repeat for three more positions of the ruler.

3. Remove the mirror and ruler. The point where the lines cross is the image position. What are the distances between the mirror and the

I

How far away is the image?

object and its image?

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Fix a plane mirror to a piece of A5 paper and draw around it.

Angle of incidence [i]

Angle of reflection [r]

angle i

angle r

Draw a normal line (at 90º) through the middle of the mirror outline.

Use a ray box to shine an incident ray at the mirror – plot the incident and reflected rays.

Measure the angles of incidence [i] and reflection [r] and record the results.

Repeat for another five angles of incidence.

The maths of reflection

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Mirrors are good reflectors but not perfect - they give two reflections.

Glass prisms are used instead of mirrors in good quality binoculars and other instruments.

Reflecting without mirrors

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Shine rays of light into a prism as shown in these ray diagrams.

Copy and complete the ray diagrams using a ruler and pencil.

Don’t forget to include arrows on your rays!

Reflecting without mirrors

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By positioning two plane mirrors at 45° to each other at either end of a tube we can make a ___________.periscope

Periscopes are used in _____________.submarines

Using plane mirrors

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1. Pale and shiny surfaces are good reflectors, dark and rough surfaces are not.

Reflection summary

2. The image in a plane mirror is laterally inverted.

3. The image is the same distance behind the mirror as the object is in front.

4. The image in a plane mirror is the same size as the object.

5. The law of reflection is:angle of incidence (i) = angle of reflection (r)

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8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour

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glass

air

The speed of light waves depends on the material they are travelling through.

air = fastest diamond = slowestglass = slower

Bending light

If light waves enter a different material (e.g. travel from glass into air) the speed changes.

This causes the light to bend or refract.

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Refraction at the air-glass boundary

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1. Place a rectangular glass block on a sheet of paper and draw around it.

2. Draw a normal line (at 90º) along the top surface of the block.

3. Shine rays of light with incident [i] angles of 30º, 60º and 0º into the block, making sure they all hit where the normal line crosses the glass surface.

Measure angle ‘r’ each time and record the results.

angle i

Refraction investigation

angle r

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Record the results of the refraction investigation in a table:

Angle of incidence [i]

Angle of refraction [r]

30º

60º

Write two ‘rules’ to describe:

what happens to the ray as it enters the glass;

what happens to the ray as it re-enters the air.

Refraction investigation – results

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airglass

When light is refracted as it travels from air to glass:angle of incidence > angle of refraction

i > r

What happens in refraction: air to glass

In general, when light rays move from a less dense medium (air) to a more dense medium (glass) they ‘bend’ towards the normal.

As the light ray travels from air into glass it moves towards the normal.

i > r

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air

glass

If the two surfaces of the block are parallel, then the ray at the start is parallel to the ray at the end.

When light is refracted as it travels from air to glass:angle of incidence < angle of refraction

i < r

What happens in refraction: glass to air

In general, when light rays travel from a more dense medium (glass) to a less dense medium (air) they ‘bend’ away from the normal.

As the light ray travels from glass into air it moves away from the normal.

i < r

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airglass

Refraction – angle of incidence = 0°

What happens to light travelling from air through a glass block when the angle of incidence is 0°?

i = 0°

When the angle of incidence is 0 the light ray is not deviated from its path.

undeviated light ray

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Refraction in a rectangular block

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Refraction – revision tip

To remember what happens to light when it is refracted, think of the word:

TAGAGA

Towards (normal)

Air

Glass

Away (from normal)

Glass

Air

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In the water.

Light moves slower through a more dense medium.

Travelling through different materials

If you were running along a beach and then ran into the water when would you be moving slower – in the water or on the sand?

Do you think light moves faster or slower in a more dense medium?

In a similar way, as light moves from one medium to another of different density, the speed of light changes.

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0306090120150180210240270300

Vac

uu

m

Wat

er

Per

spex

Speed oflight(thousandskm/s)

From this bar chart, which material do you think is denser, Perspex or water?

Perspex must be denser than water because light travels more slowly through Perspex than water.

The speed of light in different media

Light travels at 300,000 km/s in a vacuum.

As light enters denser media, the speed of light decreases.

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mud

road

Why does light change direction?

Imagine a car driving from the road into a muddy field.

In the muddy field it slows down as there is more friction.

If it enters the field at an angle then the front tyres hit the mud at different times.

Tyre 1 hits the mud first and will move more slowly than tyre 2. This causes the car to turn towards the normal.

When the car leaves the mud for the road, tyre 1 hits the road before tyre 2 and this causes the car to turn away from the normal.

tyre 1 tyre 2

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Why does light change direction?

If the car approached the muddy field at an angle of incidence of 0° then both front tyres would hit the mud at the same time.

The tyres would have the same speed relative to each other so the direction of the car would not change, it would just slow down.

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If the light enters a new medium along the normal (i.e. angle of incidence = 0) then it does not ‘bend’ because all of the light ray slows down at the s___ t___.

When light hits a medium at an angle to the n_____ the light ‘bends’ in a similar way to that described for the car in a muddy field.

Part of the light ray s____ d____ before the rest and this causes the change of d_______.

ormal

lowsirection

ime

Why does light change direction?

ame

own

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Many visual effects are caused by refraction.

Effects of refraction

This ruler appears bent because the light from one end of the ruler has been refracted, but light from the other end has travelled in a straight line.

Would the ruler appear more or less bent if the water was replaced with glass?

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The rays of light from a stone get bent (refracted) as they leave the water.

Your brain assumes these rays of light have travelled in straight lines.

Your brain forms an image at the place where it thinks the rays have come from – the stone appears to be higher than it really is.

actual location

image

Apparent depth

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The Archer fish is a predator that shoots jets of water at insects near the surface of the water, e.g. on a leaf.

image of prey

prey location

The fish does not aim at the refracted image it sees but at a location where it knows the prey to be.

The Archer fish

The Archer fish allows for the refraction of light at the surface of the water when aiming at the prey.

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Place a coin in the bottom of a bowl and clamp an empty cardboard tube so that it points above the coin.

Magic coins

Gradually add water to the bowl and watch the coin through the tube float up – can you explain this?

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1. When light bends this is called refraction.

Refraction summary

Remember that the angle of reflection [r] and the angle of refraction [r] use the same symbol.

In reflection: i = rIn refraction: i r

2. Refraction happens because the light changes speed.

3. When light enters a more dense medium (e.g. glass), it bends towards the normal.

4. When light enters a less dense medium [e.g. air], it bends away from the normal.

5. If the incident ray hits a surface at 0º, no refraction occurs.

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8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour

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Imagine you could only see in black and white.

Working in groups, each person has two minutes to give a presentation to the rest of the group about their ideas.

Life without colour

How might this affect your life?

Would it rule out any careers for you?

What dangers could there be?

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1. Shine a ray of bright white light at a prism and move the prism until colours appear.

Splitting white light with a prism

2. Draw a diagram to show what you observed.

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Splitting white light animation

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A prism splits a ray of white light into a spectrum of colours.

This is known as dispersion.

When white light is split, the colours always follow the same order.

Use this phrase to remember the order of colours:

Splitting white light into colours

Richard Of York Gave Battle In Vain

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Each of the colours of the spectrum [ROYGBIV] has a slightly different wave. What is the difference?

Each colour has a different wavelength ().

Dispersion

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The different colours of light have different wavelengths, this means they are bent (refracted) by different amounts.

Which colour is refracted the most?

Red light is refracted least because it has the longest wavelength.

Violet light is refracted the most because it has the shortest wavelength.

Dispersion

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Colours of the spectrum

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Remember how white light can be dispersed to give a spectrum of colours?

To do the opposite – two prisms are needed!

A similar effect can also be seen using a colour wheel (or Newton’s disc).

Recombining colours

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Colour in a paper or card circle with the colours of the spectrum.

Using string or a pencil spin your disc around.

What do you predict you will see?

What did you observe?

What does this tell you?

Newton’s disc

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Newton’s disc animation

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You see a non-luminous object when light hits the object and is then reflected into your eyes.

So how do we see different colours?

Why does a red dress look red?

Why does a green apple look green?

How do you see non-luminous objects such as a book?

Seeing colours

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Primary colours animation

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Colours are made by mixing other colours of light.

There are three primary colours of light used to make all other colours. What are these colours?

red green

blue

The three primary colours of light are red, green and blue.

magenta

Primary and secondary colours

The colours made by mixing two primary colours are calledthe secondary colours – magenta, yellow and cyan.

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Which primary colours?

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Why does a red snooker ball look red in white light?

White light is made up of a spectrum of colours.

The snooker ball absorbs all the colours of the spectrum except red.

Only red light is reflected into your eye, so the snooker ball appears red.

Seeing red

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Why does a green snooker ball look green in white light?

Seeing green

The snooker ball absorbs all the colours of the spectrum except green.

Only green light is reflected into your eye, so the snooker ball appears green.

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Why does a black snooker ball look black in white light?

Seeing black

The snooker ball absorbs all the colours of the spectrum.

No light is reflected into your eye, so the snooker ball appears black.

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Why does a snooker ball look in white light?

Seeing white

The snooker ball does not absorb any of the colours of the spectrum.

The whole spectrum of light is reflected into your eye, so the snooker ball appears white.

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Why does a magenta ball look magenta in white light?

Seeing magenta

The snooker ball absorbs all the colours of the spectrum except red and blue.

Red and blue light are reflected into your eye, so the snooker ball appears magenta.

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Which colours of light are reflected by these clothes?

Which colour is reflected?

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What colours are absorbed by this frog’s skin? What colours are reflected into your eyes?

This part of the skin absorbs all the colours of the spectrum except red, and so reflects red light.

Seeing different colours

This part of the skin absorbs all the colours of the spectrum and none are reflected.

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What colours are absorbed by this flower? What colours are reflected into your eyes?

Seeing different colours

This part of the flower absorbs all colours except red and green. It reflects red and green light, and so appears yellow.

This part of the flower absorbs no colours. It reflects them all and so appears white.

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Filters let certain colours of light pass through, but absorb all other colours.

Using different coloured filters placed in front of your eye, look around the classroom and see what effect they have on your vision.

object filter

Using coloured filters

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A red filter absorbs all colours…

A blue filter absorbs all colours…

A green filter absorbs all colours...

…apart from red light.

…apart from blue light.

…apart from green light.

Red, blue and green filters

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A magenta filter absorbs all colours…

A cyan filter absorbs all colours…

A yellow filter absorbs all colours...

…apart from red and blue.

…apart from green and blue.

…apart from red and green.

Magenta, cyan and yellow filters

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Using colour filters

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Why do colours look different in different coloured light?

Consider a red ball in red light.

The red light shines on the ball.

The red ball reflects the red light and so appears red.

Seeing colours in coloured light

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What colour does a red ball appear in green light?

Seeing colours in coloured light

The green light shines on the ball.

The red ball only reflects red light and so it absorbs the green light.

So in green light, this ball does not reflect any light and so appears black.

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What colour does a green ball appear in blue light?

Seeing colours in coloured light

The blue light shines on the ball.

The green ball only reflects green light and so it absorbs the blue light.

So in blue light, this ball does not reflect any light and so appears black.

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What happens when using a coloured filter which lets through more than one type of light?

What will a red ball look like in magenta light?

Seeing colours in coloured light

The magenta light, which is a mixture of red and blue light, shines on the ball.

The red ball only reflects red light and so absorbs the blue light.

So in magenta light, this ball reflects the red light and appears red.

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The next two slides include a girl wearing a t-shirt and trousers.

The girl is standing in a different coloured light each time. The colour of this light is written at the top of the slide.

The aim of each activity is to decide what colours the girls’ clothes would appear in each type of coloured light.

Drag the correct t-shirt and trousers onto the girl to find out if you have selected the correct colours.

Coloured light activity – instructions

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Coloured light activity 1

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Coloured light activity 2

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How would the colours in this flag appear under these lighting conditions?

Flag colours in different coloured light

a) red light b) green light

c) blue light

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Object (Colour) Colour Filter Appearance

red ball red

red ball blue

blue book green

blue book magenta

green apple cyan

green apple magenta

red and blue tie red

red

black

black

blue

green

black

red and black

What colour does each object appear under the given lighting conditions?

What colour does it appear?

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8K Light

Contents

What is light?

Reflection

Refraction

Summary activities

Colour

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Glossary

absorption – When light is taken in by a material.dispersion – The separating of the colours in light,

e.g. when white light passes through a prism.image – A copy of an object formed when light is reflected

from a mirror.light – A form of energy that is detected by the eyes. law of reflection – When light is reflected, the angle of

incidence equals the angle of reflection.prism – A block of glass, usually triangular, which separates

the colours in light.reflection – The bouncing back of light from a surface.refraction – The bending of light when it passes into a

different medium.spectrum – The range of colours that make up white light.

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Anagrams

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Multiple-choice quiz