8 k light (boardworks)

KS3 Physics

8K Light
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© Boardworks Ltd 2004
2005

Contents

8K Light
What is light?
Reflection
Refraction
Colour
Summary activities
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What is light?
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.

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Which materials let light through?
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.
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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Which materials let light through?
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.
Material
Tracing paper
1 ply tissue
2 ply tissue
Smoked glass
Glass
Perspex
Wood
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5

Opacity (%)

Shading chart
100%
75 %
50 %

2.5 %
5%
10 %

30 %
15 %
25 %

20 %

0%
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How does light travel?
1. Fill a clear glass trough or empty fish tank with smoke.
2. Use a slit to shine rays of light through the tank and
describe what you see.

Light waves travel in straight lines.

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Seeing light
How do you see an object?
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.

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Seeing a luminous object
A luminous object gives out light and can also be called
a light source.
How does light from a light bulb and other light sources
reach your eye?

Light travels in a straight line directly into your eye.
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Seeing a non-luminous object
Objects that do not give out light are non-luminous.
How does your eye see non-luminous objects such as a
book?

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

8K Light
What is light?
Reflection
Refraction
Colour
Summary activities
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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.

Objects that do not reflect light well:
 have rough, matt surfaces and are usually dark colours.
 give no or diffuse images because they reflect the light
irregularly.

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Good and bad reflective materials
Arrange these items along the arrow:

yellow
banana

r ed
roses

white
pape
r

polished
black
shoes

aluminium
foil
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tarm
ac
road

best
reflectors

blue
car
angerin
t

tree
bark

e

gree
n
leaf

worst
reflectors
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Light that is not reflected
What happens to 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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Reflection investigations
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.

1. Reading in mirrors.
2. How far away is the image?
3. The maths of reflection.
4. Reflecting without mirrors.
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Reading in mirrors – instructions
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.
Put your results in a table like this:

Name
Natasha

46

Pashmina

56

David
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Time taken to read
(s)

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Reading in mirrors – words

dog

ball

bat

bike

ants

park

fins

pink

litter

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man

sandy

shark

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Reading in mirrors – results
1. Who read the words in the quickest time?
2. Plot a bar chart of your results:

Time taken
[s]

A graph showing the results of 'Reading in
Mirrors'
100
50
0
Natasha

Pashmina

David

Name

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

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Lateral inversion
A plane mirror reflects light regularly so that it produces a
clear image which is the same size as the object.
What is different about the image?

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

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How far away is the image?
1. Fix a plane mirror
along the centre of a
piece of A4 paper and
draw around it.

I

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.
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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
object and its image?
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The maths of reflection
Fix a plane mirror to a piece of A5 paper and draw around it.
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.

angle i
angle r
Angle of
Angle of
incidence [i] reflection [r]

Repeat for another five
angles of incidence.
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Reflecting without mirrors
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.

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Reflecting without mirrors
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!

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Using plane mirrors
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

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

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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Contents

8K Light
What is light?
Reflection
Refraction
Colour
Summary activities
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Bending light
The speed of light waves depends on the material they
are travelling through.
air = fastest

glass = slower

diamond = slowest

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.

air
glass

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Refraction at the air-glass boundary

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

angle i
angle r

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.

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Refraction investigation – results
Record the results of the refraction investigation in a table:
Angle of
incidence [i]

Angle of
refraction [r]

30º
60º
0º

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.
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What happens in refraction: air to glass
When light is refracted as it travels from air to glass:
angle of incidence > angle of refraction
∠i > ∠r
As the light ray travels from
air into glass it moves
towards the normal.
In general, when light rays
move from a less dense
medium (air) to a more dense
medium (glass) they ‘bend’
towards the normal.

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∠i > ∠r
air
glass

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What happens in refraction: glass to air
When light is refracted as it travels from air to glass:
angle of incidence < angle of refraction
∠i < ∠r
As the light ray travels from
glass into air it moves away
from the normal.
In general, when light rays travel
from a more dense medium (glass)
to a less dense medium (air) they
‘bend’ away from the normal.
If the two surfaces of the block are
parallel, then the ray at the start is
parallel to the ray at the end.
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glass
air
∠i < ∠r

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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.

air
glass

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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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?
In the water.
In a similar way, as light moves from one medium to
another of different density, the speed of light changes.
Do you think light moves faster or slower in a more dense
medium?
Light moves slower through a more dense medium.

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The speed of light in different media

Perspex

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

Speed of
light
(thousands
km/s)

Water

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

300
270
240
210
180
150
120
90
60
30
0
Vacuum

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

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

tyre 1

tyre 2

mud

2005

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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 When light hits a medium at an angle to the n_____
ormal
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
lows own
this causes the change of d_______.
irection
 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___.
ame ime

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Effects of refraction
Many visual effects are caused by 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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Apparent depth
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.
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image

actual location

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The Archer fish
The Archer fish is a predator that shoots jets of water at
insects near the surface of the water, e.g. on a leaf.
The Archer fish allows
for the refraction of light
at the surface of the
water when aiming at
the prey.

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.

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

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

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Refraction summary
1. When light bends this is called refraction.
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.
Remember that the angle of reflection [r] and the
angle of refraction [r] use the same symbol.
In reflection: i = r
In refraction: i ≠ r
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Contents

8K Light
What is light?
Reflection
Refraction
Colour
Summary activities
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Life without colour
Imagine you could only see in black and white.
How might this affect your life?
Would it rule out any careers for you?
What dangers could there be?

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

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

2. Draw a diagram to show what you observed.

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Splitting white light animation

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Splitting white light into colours
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:

Richard Of York Gave Battle In Vain
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Dispersion
Each of the colours of the spectrum [ROYGBIV] has a
slightly different wave. What is the difference?

Each colour has a different wavelength (λ).
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Dispersion
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.
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Colours of the spectrum

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Recombining colours
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).

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Newton’s disc
Colour in a paper or card circle with the colours of the
spectrum.

Using string or a pencil
spin your disc around.

What did you observe?

What do you predict you
will see?

What does this tell you?

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Newton’s disc animation

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Seeing colours
How do you see non-luminous
objects such as a book?
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?
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Primary colours animation

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Primary and secondary colours
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?

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

green

red

magenta
blue
The colours made by mixing two primary colours are called
the secondary colours – magenta, yellow and cyan.
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Which primary colours?

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Seeing red
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.

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

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

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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Seeing white
Why does a

snooker ball look

in white light?

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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Seeing magenta
Why does a magenta ball look magenta in white light?

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 colour is reflected?
Which colours of light are reflected by these clothes?

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Seeing different colours
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.
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This part of the skin
absorbs all the colours
of the spectrum and
none are reflected.
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Seeing different colours
What colours are absorbed by this flower?
What colours are reflected into your eyes?

This part of the flower absorbs
all colours except red and green.
It reflects red and green light,
and so appears yellow.
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This part of the flower
absorbs no colours.
It reflects them all
and so appears white.
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Using coloured filters
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

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filter

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Red, blue and green filters
A red filter absorbs
all colours…
…apart from red light.
A blue filter absorbs
all colours…
…apart from blue light.
A green filter absorbs
all colours...
…apart from green light.
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Magenta, cyan and yellow filters
A magenta filter absorbs
all colours…
…apart from red and blue.
A cyan filter absorbs
all colours…
…apart from green and blue.
A yellow filter absorbs
all colours...
…apart from red and green.
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Using colour filters

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Seeing colours in coloured light
Why do colours look different in different coloured light?
Consider a red ball in red light.

The red light
shines on
the ball.

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

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What colour does a red ball appear in green 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?

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?

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

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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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Coloured light activity – instructions
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.

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Coloured light activity 1

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Coloured light activity 2

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Flag colours in different coloured light
How would the colours in this flag appear under these
lighting conditions?
a) red light
b) green light
c) blue light

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What colour does it appear?
What colour does each object appear under the given
lighting conditions?
Object (Colour) Colour Filter
red ball
red ball

blue

blue book

green

blue book

magenta

green apple

cyan

green apple

magenta

red and blue tie

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red

red

Appearance

red
black
black
blue
green
black
red and black
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Contents

8K Light
What is light?
Reflection
Refraction
Colour
Summary activities
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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

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