PW8
SCIENCE FOR PRIMARY TEACHERS
TheOpen
University
STUDY COMMENTARY FOR UNlT 10:
MODELLING THE BEHAVIOUR OF LIGHT
ATTAINMENT TARGET ADDRESSED IN
UNlT 10: AT15
5 WHAT KIND OF WAVE?
ATTAINMENT TARGET ADDRESSED IN
Study notes
Teaching notes
SUPPLEMENT: AT14
Key points
COMMENTARY GUIDE
6 LIGHT AS PARTICLES
Study notes
Teaching notes
Key points
1 INTRODUCTION
AT 15
Study notes
Teaching notes
Key point
7 LIVING WITH THE WAVE-PARTICLE
DUALITY
8 TV NOTES: LIGHT-IN SEARCH OF
A MODEL
2 WHY A WAVE MODEL ANYWAY?
AT15: levels 1 to 5
Study notes
Teaching notes
Key points
Study notes
Teaching notes
Key points
SUPPLEMENT: SOUND
3 WAVE CONCEPTS
Study notes
Teaching notes
Key points
AT14: levels 1 to 5
Study notes
Teaching notes
Key points
4 DIFFRACTION AND THE WAVE
MODEL
RESOURCES
AT15: levels 1 to 5
Study notes
Teaching notes
Key points
QUESTIONS
NOTES
20
CENTRE FOR SCIENCE EDUCATION
STUDY COMMENTARY FOR UNlT 10
ATTAINMENT TARGET
ADDRESSED IN UNlT 10: AT15
AllAlNMENT TARGET 15: USING LIGHT AND
ELECTROMAGNETIC RADIATION
Pupils should develop their knowledge and understanding of the properties and
behaviour of light and electromagnetic waves.
Key
stage
1
Programme of study
Children should have opportunities to
explore a variety of light sources and effects
related to shadows, reflection and colour.
Level
Statement of attainment
Pupils should
1
know that light comes from different sources
be able to discriminate between colours and
match them or, where appropriate,
demonstrate an understanding of colour in the
environment
know that light passes through some
materials and not others, and that when it
does not, shadows may be formed
be able to draw pictures, showing features
such as light, colour and shade
2
Children should explore light passing
through different transparent objects, for
example, lenses, colour filters, water,
p r i s m s . Children should carry out
investigations on shadows and the
formation of images in mirrors and other
devices. Children should represent in
drawings and diagrams their ideas about
how light behaves in terms of light, colour
and shade.
3
know that light can be made to change
direction and shiny surfaces can form images
be able to give an account of an investigation
with mirrors
know that we see objects because light is
scattered off them and into our eyes
know that light travels in straight lines and
use this to explain the shapes and sizes of
shadows
understand how light is reflected
SCIENCE FOR PRIMARY TEACHERS
ATTAINMENT TARGET
ADDRESSED IN SUPPLEMENT: AT14
AllAlNMENT TARGET 14: SOUND AND MUSIC
Pupils should develop their knowledge and understanding of the properties,
transmission and absorption of sound.
Key
stage
1
Programme of study
Children should have the opportunity to
experience the range of sounds in their
immediate environment and to find out
about their causes and uses. They should
investigate ways of making and
experiencing sounds by vocalizing and
striking, plucking, shaking, scraping and
blowing, for example using familiar objects
and simple musical instruments from a
variety of cultural traditions. Children
should explore various ways of sorting
these sounds and instruments.
Children should be made aware of the way
sound is heard and that sounds, including
musical notes, are made in a variety of ways
and can be pleasant or obtrusive in the
environment. They should explore the
changes in pitch, loudness and timbre of a
sound, for example by changing the length,
tension, thickness or material of a vibrating
object, and through ways of causing sound,
for example use of different mallets,
overblowing.
Level
Statement of attainment
Pupils should
know that sounds can be made in a variety of.
ways
know that sounds are heard when the sound
reaches the ear
be able to explain how musical sounds are
produced in simple musical instrument
know that sounds are produced by vibrating
objects and can travel through different
materials
be able to give a simple explanation of the
way in which sound is generated and can
travel through different materials
know that it takes time for sound to travel
understand that the frequency of a vibrating
source affects the pitch of the sound it
produces
understand the relationship between the
loudness of a sound and the amplitude of
vibration of the source
understand the importance of noise control in
the environment
STUDY COMMENTARY FOR UNIT 10
TABLE 1 Levels of the attainment targets addressed in Unit 10 and Supplement
ATs
Level 1
a
1
2
3
4
5
6
9
10 1 1 12 13 1 4 15 16
0
0
b
C
Level 2
e
f
Note: a, b, c, etc. refer to the statements of attainment. For the complete statements,
please see pp. 3 and 4.
SCIENCE FOR PRIMARY TEACHERS
COMMENTARY GUIDE
The main attainment target that is related to Unit 10 is AT15.
As the title suggests, Unit 10 is about modelling the behaviour of light. Its
major aim is to present and justify scientific models that represent the way that
light behaves rather than to describe in detail the many important properties of
light. Because of this, the Unit does not teach some of the very basic facts that
are relevant to AT15, levels 1 to 5; for example, about sources of light, how we
see an object, how an image is formed with a mirror, and so on. To fill this gap,
the Study notes for Section 2 include a brief resumt of some of these basic facts.
This Unit is probably the most demanding so far. Though you should read
through the complete Unit in order to get an overall picture of the dual waveparticle model, you can skim through Sections 4.3.2 to 4.4.3 and Sections 6.1
to 6.4 if you are short of time. It is important that you do the Experiment in
Section 2 because the discussion in Section 4 is based on the observations you
make in this Experiment.
Sound is an important part of the national curriculum in science for primary
schools, but because of space limitations the Science Foundation Course does
not teach this subject. At the end of this Study Commentary you will find a
Supplement which briefly covers the facts about sound that are most relevant for
key stages 1 and 2 of AT14 Sound and music, and provides some ideas for
presenting this material in the classroom.
1 INTRODUCTION
Main attainment target addressed in Section 1: AT15
STUDY NOTES
This short Section sets up the theme of the Unit-modelling the properties of
light-and introduces the two models that are used: the wave model and the
particle model. It is important to bear in mind throughout the Unit that both of
these are models that are used to help us visualize the way that light behaves.
Light isn't simply a wave like a water wave or a sound wave, but in many
circumstances it behaves as though it were. Nor is light simply a collection of
discrete particles carrying energy and momentum, though in other circumstances
we can understand its behaviour in those terms.
TEACHING NOTES
There are no Teaching notes for this Section.
KEY POINT
The concept of modelling is once again introduced, and its importance
demonstrated, to show how we can begin to understand the properties and
behaviour of light. This time two models are needed-a wave model and a
particle model. For this reason one talks of the wave-particle duality of
light.
STUDY COMMENTARY FOR UNIT 10
2 WHY A WAVE MODEL
ANYWAY?
Main attainment target and levels addressed in Section 2: AT15: levels 1
t o 5
STUDY NOTES
It is important to perform the Experiment associated with this Section, since the
observations that you make will form the basis of the discussion of diffraction in
Section 4.
This Section demonstrates that light behaves in a similar fashion to other waves,
such as water waves, sound waves and seismic waves. The next three Sections
develop the basic physics of waves, and describe in more detail the wave nature
of light. However, many of the elementary properties of light related to the first
five levels of AT15 can be discussed without explicitly referring to waves, and
this will be done briefly below.
LIGHT SOURCES
Light radiates out from sources (e.g. the Sun, light bulbs) along straight-line
paths (Figure 1). These paths are usually referred to as light rays. The rays
represent the paths travelled by the light waves. For a small source of light, the
rays in effect spread out radially from a single point (Figure la). For an extended
source, however, such as a fluorescent tube, the light spreads radially from each
point on the source (Figure lb).
fluorescent tube
FIGURE 1 (a) For a small source, the light rays spread out along radial paths.
(b) For a large source, the light rays spread out radially from each point on the source.
TRANSPARENT AND OPAQUE OBJECTS, AND SHADOWS
Transparent materials, such as glass, are materials that light can pass through.
Materials that light cannot pass through, such as metals and wood, are said to be
opaque. Shadows are formed when strong light from one direction is blocked by
opaque objects. The positions and sizes of shadows can be understood by drawing
the light rays from a source that pass the edges of an object (Figure 2), as was
done in Units 1 and 2 when eclipses were discussed.
SCIENCE FOR PRIMARY TEACHERS
light
from
large light source
FIGURE 2 (a) Rays of light from the Sun casting a shadow on the ground; the
length of the shadow depends on the Sun's elevation in the sky. (bj Light rays
diverging from a small, nearby source produce a magnified shadow. (c) With a larger
or extended light source, rays from different points cast shadows in different places;
check for yourself that it is only the small central area of the shadow that receives no
light at all from the source.
SEEING
LIGHT SOURCE
We see a light source when rays of light from the source enter our eyes (Figure
3a). If an opaque object is interposed between the source and our eyes, then we
can no longer see the source (Figure 3b).
STUDY COMMENTARY FOR UNIT 10
FIGURE 3 (a) Light from the bulb travels along straight lines (light rays) to the
eye: we see the bulb. (b) Block the direct path of rays from bulb to eye and the bulb
can no longer be seen.
SEEING NON-LUMINOUS OBJECTS
Most objects are non-luminous-they do not emit light. We can see objects
only when light from a source (e.g. the Sun, a light bulb) is scattered from the
surface of the object into our eyes (Figure 4). If no light is allowed to reach nonluminous objects, then we cannot see them.
FIGURE 4 To read a book, light from some source must be scattered and some of
the scattered rays must enter the eye.
REFLECTION
When light hits a smooth, highly polished surface it is not scattered in all
directions. It is reflected in the way shown in Figure 5a (overleaf): the angle of
incidence i and the angle of reflection R are equal. In Units 5-6 you saw that
seismic waves are reflected in this way.
SCIENCE FOR PRIMARY TEACHERS
left hand
left hand
reversed in
mirror
FIGURE 5 (a) Reflection of a ray of light from a mirror: the angles i and R are
equal. (b) Reflected light apparently diverging from an image of the light source.
(c) The reflection of a hand in a mirror: the reflection of a left hand appears to be a
right hand.
When light from a small source is reflected from a plane mirror, all the reflected
light appears to diverge from a point behind the mirror (Figure 5b). Looking at
the mirror, the apparent source-the image of the source-appears to be the
STUDY COMMENTARY FOR UNIT 10
same distance behind the mirror as the real source is in front of the mirror. This
is true for any object in front of a plane mirror: there appears to be an image of
the object the same distance behind the mirror (Figure 5c). If the mirror is not
plane but curved; then the image will be a distorted version of the object, as you
have probably seen when visiting a 'hall of mirrors', or when looking at your
reflection in a spoon or a polished metal teapot.
REFRACTION
A ray of light changes direction-it is refracted-when it travels across a
boundary between two transparent materials, such as air and water (Figure 6).
This happens because the speed of light is different in different materials. You
may remember from Units 5-6 that seismic waves are also refracted at a
boundary where their speed changes. The equation describing the change of
direction is the same for light and for seismic waves:
sin i - V-l
-sin r
VZ
where i and r are the angles of incidence and refraction (Figure 6), and v , and v,
are the speed of the wave before and after it crosses the boundary. For light
travelling from air into water,
Now, light travels faster in air than in water (or in glass), so v
than 1. In mathematical notation
~ ~ , is~ greater
Z ) ~ ~ ~
(Remember, the symbol > means 'greater than'.) From Equation 1, this implies
that
that is, that
Hence, the angle of incidence i is greater than the angle of refraction r, and so the
light is refracted towards the normal (Figure 6).
FIGURE 6 A ray of light is refracted when it crosses an interface between two
transparent materials. The angle between the ray and the normal (a line perpendicular
to the interface) is smaller in the material in which the speed of light is slower.
SCIENCE FOR PRIMARY TEACHERS
Refraction accounts for the apparent bending of a stick in water and for water
appearing to be shallower than it really is (Figure 7a). Refraction is used to bend
light with prisms (Figure 7b) and to focus light with lenses (Figure 7c).
glass prism
FIGURE 7 (a) Light from the stick is refracted away from the normal as it crosses
the air-water boundary, so the eye perceives the submerged part of the stick to be
higher than it really is. (b) Refraction of light by a prism: note again that the angles
between the rays and the normal in the glass are smaller than the angles in air.
(c) Refraction of parallel light by a converging lens brings the light to a focus.
STUDY COMMENTARY FOR UNIT 10
INTERNAL REFLECTION
When light meets an interface between two different transparent materials, some
of it is reflected as if from a mirror, and the rest is refracted as described above. If
the speed of light is higher in the second material than in the first material, and
if the angle of incidence is large enough, all the light can be reflected. You saw
in Units 5-6, Section 2, that seismic waves can be trapped by total internal
reflection within a medium in which they have low speed. In the same way,
light waves can be trapped within glass or plastic, where their speed is lower
than in air. Optical fibres, used in medical endoscopes, telecommunications and
in some decorative lighting, work on this principle. The fibres trap the light
within them and it can emerge only at the end, however much they are twisted
and bent (Figure 8).
thin glass
or plastic fibre
FIGURE 8 Light is trapped within an optical fibre by total internal reflection.
DIFFRACTION
Using rays to represent the way that light travels works well in all the situations
described above. However, when light passes through very small apertures, such
as the slits and gratings supplied with PS548, its behaviour is more
complicated. The next two Sections develop the wave concepts that are needed to
explain the observations of diffraction made with the slide provided.
TEACHING NOTES
The programmes of study for key stages 1 and 2 and the statements of
attainment for levels 1 to 5 aim to give children experiences that will widen
their understanding of light. They should discover how light from a variety of
sources behaves, and investigate various effects related to shadows, reflection,
colour, the passage of light through transparent materials and the formation of
images in mirrors and other shiny objects.
BUILDING ON CHILDREN'S EXISTING IDEAS
Children start to make sense of the world around them from the moment they are
born, so they will already have begun to form their own ideas to explain various
phenomena. Sometimes their explanations are perfectly logical according to the
experiences they have had to date, and the teacher's task is to lead them, using
first-hand practical experience where possible, towards accepted scientific
explanations. This restructuring of ideas can be made easier if the conceptual
leap towards the new idea is not too great.
SCIENCE FOR PRIMARY TEACHERS
THE TEACHER'S TASK
You may feel after reading Section 1 of Unit 10 that all your experience of the
behaviour of light to date has nothing to do with either wave or particle models
and that we are asking you to take an idea as an act of faith, something that is
inconsistent with good teaching and learning styles.
Fortunately the national curriculum does not require primary school children to
understand wave or particle models, or the interaction of electromagnetic
radiation with matter!
What is required of the teacher is to give the children high-quality learning
experiences and help them to observe carefully, and to communicate their
observations in an appropriate manner. Then, when they do meet wave and
particle theories later on, their observations will be put into context and the
children will be quite clear about the phenomena that are being modelled.
Key stages 1 and 2 deal only with those properties and behaviours of light that
can be explained by the wave model. The programme of study for key stage 2
expects children to explore the passage of light through different transparent
objects, but refraction is not assessable until level 6 (secondary).
LIGHT AND DARK
All sighted children will, of course, have experience of night and day (see Study
Commentary for Unit 2)-but have they talked about darkness and light? Light
is very much taken for granted, except when there is a power cut, and confusion
is caused for some children because we talk about light and dark colours.
Do the children know that darkness is the absence of light? Have they ever
experienced real darkness? Many town-dwelling children have not seen the stars
in a dark sky because of the sodium glare of the street lighting, and many sleep
with a light on or with their bedroom door open.
Can they make a really dark place, say under a table draped with a heavy
tablecloth or blanket? Is it really dark inside or can they see light below the
cloth?
Where does light come from? What happens at night? If a child has not yet
learned about reflection, will you treat the Moon as a source of light at night?
Can they watch television in a dark room? Can they see in the dark? What can
they see in the dark? Light sources can be seen in the dark, but other things
cannot, except by reflected light. Colours cannot be seen in very dim light, but
shapes, i.e. silhouettes, can.
What do we see with? This is not always obvious to young children.
PASSAGE OF LIGHT THROUGH MATERIALS
One way of introducing level 2-'Pupils should know that light passes through
some materials and not others.. .'-is to collect packaging materials and to sort
them out into those you can see through and those you cannot. Some wrappers
are intriguing because you cannot see the items wrapped up in them, but if you
hold the empty wrappers to your eyes and look towards the light you can
actually see through them.
Ask the children which materials they think they will be able to see torchlight
through, then let them test their predictions.
SHADOWS
The formation of shadows needs a strong light source. A torch is usually
inadequate but will sometimes work if you make a screen from greaseproof or
tracing paper to 'catch' the shadow.
Can the children make coloured shadows? Can they make the shadows bigger or
smaller? Can they make a shadow-puppet theatre?
STUDY COMMENTARY FOR UNIT 10
Working outside on a bright sunny day can be fruitful, especially with younger
children. Do the children realize what conditions are necessary for them to have a
shadow? What happens if a cloud covers the sun? How many shadows have they
got? (Contrast this, if possible, with a footballer playing on a floodlit pitch.)
Does their shadow point in a different direction when they turn around? Are their
shadows on the same side of them as the Sun? Look at the shadows of other
things outside. Do they all point in the same direction? Why do they think that
is? Does a clear plastic bag have a shadow? Do coloured things look black in a
shady place? How do they look?
This work can be incorporated into work on shadows included in the Study
Commentary for Unit 2.
DRAWING PICTURES
Drawing pictures showing features such as light, colour and shade is one way of
recording, and might also be used as a basis for assessment. Will it enable you
only to assess their understanding of light, colour and shade or are other factors
involved? You may like to discuss this during a tutorial session.
REFLECTIONS
Making a collection of shiny things, both packaging materials and common
artefacts such as spoons and metal teapots, can lead into level 3. Young children
do not always understand the word 'shiny', even if English is their mother
tongue, so this understanding needs to be established first.
Which shiny things can they see through? Can they see their faces or hands 'in'
some of the things? Do their faces or hands look the same in all shiny things or
are they different in any way? Which shiny things alter their appearance mostflat (plane) shiny surfaces or curved ones? Do they look fatter? thinner? upsidedown? Does the image alter in any way if the shiny surface is brought nearer or
moved further away?
Children do not always realize that the images of objects (and of themselves) are
'laterally inverted' by plane mirrors. They can be questioned about models they
have built using coloured plastic bricks on mirrors placed flat on the table. Work
on symmetry can be pursued in conjunction with work on mirrors.
Older children often enjoy writing secret messages in mirror-writing.
Why do the children think the word 'ambulance' appears as a mirror-image on
the front of some ambulances?
Can they use a mirror to see behind themselves without turning their heads? to
see under the table without bending down? to see on to the top of a tall
cupboard?
Can they make a 'patch' of sunlight on the classroom wall or ceiling using a
mirror? (REMEMBER THAT CHILDREN MUST NOT LOOK
DIRECTLY AT THE SUN.)
What can they see behind them if they look into a convex or a concave mirror?
How does this compare with what they see in a plane mirror of about the same
size?
MULTIPLE REFLECTIONS
What do they see when they place a small object (a coin, a plastic cube, etc.) in
the angle formed by taping two plane mirrors together? Does this alter when
they make the angle bigger or smaller? Why do they think this is? Top infants
and junior school children may proffer explanations for this. Some kaleidoscopes
are constructed in such a way that you can see the angled mirrors from one end.
SCIENCE FOR PRIMARY TEACHERS
REFRACTION
The level 3 statement of attainment 'Pupils should know that light can be made
to change direction.. .' could refer to the bending of light by refraction when it
travels through glass prisms, water, etc. The most common demonstration of
this is the 'bent' pencil in a jar of water. Another effective way to demonstrate
this phenomenon is to place a clear plastic fish tank containing water on a sheet
of newspaper. When viewing the newspaper from above, the part that is below
the tank, and so is seen through the water, appears to be higher than the rest of
it, which is viewed directly through the air. You could also refer the children to
the lane lines at the bottom of the swimming pool viewed through still water
and when the water is moving.
SEEING THINGS BY SCATTERED LIGHT
The level 4 statement 'Pupils should know that we see objects because light is
scattered off them and into our eyes' is a very hard concept for children to grasp.
Although they usually understand that we need light in order to see things and
that we cannot see things when it is really dark, many children think we see
things because 'rays' of some sort leave our eyes and 'hit' the objects. It is very
difficult to prove to them that this is not so because many of our everyday
observations could be explained by that theory. For example, we can see a dog
in daylight if there is nothing solid between us and the dog; however, if it is on
the other side of a tall wall we will not be able to see it (although we might be
able to hear it). This experience can easily be explained by the 'rays-from-oureyes' theory-we cannot see the dog because the wall stops the rays from our
eyes reaching the dog.
Sometimes on a brilliantly sunny day it can be quite uncomfortable to look at a
piece of white paper or a white painted door. Perhaps consideration of why this
is so could lead children to realize that it is not only shiny objects that reflect
light. Also, thinking about the fact that in a completely dark place we cannot
see a piece of white paper at all should lead to the realization that when we do
see the white paper it is because it is reflecting light into our eyes. The sort of
reflective material used on clothing for road-safety purposes might be worth
investigating. It does not reflect light in the same way as a mirror but it does
show up even in very poor light.
LIGHT TRAVELLING IN STRAIGHT
The second statement from level 4, 'Pupils should know that light travels in
straight lines and use this to explain the shapes and sizes of shadows', also
demands quite a conceptual leap for children. Although they can be reminded that
we cannot see around comers with the naked eye, they are not usually aware that
light travels. In everyday life we do not see individual rays of light but just
general lightness in a lit-up place. The children will need practical opportunities
to use beams of light to observe that these have straight edges and do not waft
about like plumes of smoke. Children could be asked to draw the beams of light
seen from car headlamps in fog or rain, or sunbeams shining through a window
and showing up in misty air. Such beams always have perfectly straight edges.
Investigations of shadows using measurements of sizes and distances may help
them come to some conclusions about light travelling in straight lines. The
inverted image you obtain if you make and use a pinhole camera can also help
older children to understand that light travels in straight lines.
If you can obtain a piece of fibre-optic cable, children could investigate shining a
torch down it and directing the light round corners+ontrast this with the
ordinary behaviour of light travelling in straight lines.
Science for ages 5 to 16 (Department of Education and Science and the Welsh
Office, 1988) defined 'understand' as being able to apply and use knowledge in a
new, given situation. The level 5 statement 'Pupils should understand how light
is reflected' therefore implies that children should be able to use this knowledge
for a purpose, such as solving problems.
STUDY COMMENTARY FOR UNIT 10
KEY POINTS
Light shows similar properties to waves: it transfers energy from one place
to another, and it is reflected, refracted and diffracted in the same way as
waves.
Properties of light, such as shadow formation, reflection and refraction, are
amenable to simple experimental investigations in the primary classroom.
WAVE CONCEPTS
Main attainment target and levels addressed in Section 3: none
STUDY NOTES
The discussion of waves in this Section is important because of its generality. It
applies to all forms of waves: light, sound, water, seismic, and so on.
Section 3 concentrates on water waves because they can be seen directly: speed,
wavelength, frequency, period and amplitude can all be measured with a ruler and
a stop-watch. But the basic ideas introduced here apply equally well to all waves,
of whatever type. Typical values of v, A, f,T and A will vary greatly from one
type of wave to another. For example, Table 2 compares the water wave
described in SAQ 2 with a typical seismic P-wave described in Units 5-6.
TABLE 2 Comparison of a water wave and a seismic P-wave
Water wave
Seismic P-wave
-
5.1 m s-I
8.1 x 1 0 3 m s-I
17 m
1.4 x l o 3 m
frequency, f
0.03 H z
5.6Hz
period, T
3.3 s
0.18 s
speed, v
wavelength,
A
You should remember from Units 5-6 that a P-wave is a pressure wave, so in
that case a pressure variation is travelling through the Earth rather than a height
variation as in a water wave. You can't see this pressure variation, but the
vibrations of the Earth associated with the P-wave can certainly be felt!
The ideas discussed in Section 3 about waves passing through each other and
about superposition are also common to all kinds of waves. For example, light
waves don't knock one another off course (fortunately!). The light waves
travelling from this page to your eyes have to cross the paths of many other
light waves, yet you can see the page as clearly as you would if light from all
other sources were carefully screened out. In addition, constructive and
destructive superposition can be observed with all types of wave. Such
superposition with light is not something that we are normally aware of. The
conditions have to be just right-remember, you need two waves with the same
wavelength and amplitude. That is just what was arranged in the double slit
experiment in Section 2; you will see in the next Section how the bright and
dark fringes that you observed can be explained in terms of constructive and
destructive superposition.
TEACHING NOTES
There are no Teaching notes for this Section.
SCIENCE FOR PRIMARY TEACHERS
KEY POINTS
The language of waves, which was developed in this Section to describe
water waves, can be used to describe all types of wave.
The important terms in the vocabulary of this wave language are amplitude,
wavelength, frequency, period and wave speed.
Two waves of the same type can be superposed, and when the waves have
the same wavelength and amplitude this superposition can be constructive
or destructive.
4 DIFFRACTION AND THE WAVE
MODEL
Main attainment target and levels addressed in Section 4: AT15: levels 1
to 5
STUDY NOTES
This Section uses the wave model to explain the observations made during the
Experiment in Section 2.2. The main ideas are developed in the first half of this
Section, and you should study up to the end of Section 4.3.1 carefully. The rest
of the Section develops the mathematics of diffraction by a double slit and by a
diffraction grating-if pressed for time you could skim quite quickly through
Sections 4.3.2-4.4.2. Section 4.4.3 discusses colour; you should read this
carefully.
LIGHT AND COLOUR
Visible light covers the wavelength range from approximately 400nm (i.e.
400 x 10-9 m, or 4 x 10-7 m) to about 700 nm; the colours of light in
different parts of this range are indicated in Figure 9. White light is a
combination of light with wavelengths throughout the visible range. The white
light from a light bulb can be separated into the different wavelength
components using a diffraction grating (as in the Experiment) or more simply
with a beam of light and a prism (Figure 10). An even simpler way of producing
coloured light is by using transmission filters that transmit certain ranges of
wavelengths and absorb all others. For example, a red filter will transmit light at
the red-orange end of the spectrum but absorb light in the region from yellow to
violet. A filter that removes the central region of the spectrum (from orange to
green) will appear purple because this is the result of combining the red, blue
and violet light that is transmitted.
00
A lnm
500
11
1
orange
yellow
green
blue
indigo
FIGURE 9 The colours corresponding to wavelengths in the visible region of the
electromagnetic spectrum.
STUDY COMMENTARY FOR UNIT 10
prism
FIGURE 10 White light can be separated into a rainbow of colours by passing it
through a glass prism.
The colours of objects around us are explained in a similar way to the colours of
filters, except that the light reaching our eyes from an object is generally
reflected light rather than transmitted light. Thus a green apple appears green
because it reflects light from the green region of the spectrum and absorbs light
from both the red-orange and the blue-violet ends of the spectrum.
It is important to note the difference between mixing coloured lights (as in stage
lighting, or on a TV screen) and mixing coloured paints. When mixing light we
add to the range of wavelengths present. So combining red light and green light
gives (red + green) light, and this is actually perceived as yellow by the brain,
even though there may be no wavelengths present corresponding to the yellow
region of the spectrum. On the other hand, when mixing paint we reduce the
range of wavelengths present because each of the paints absorbs a different range
of wavelengths. Thus if a red paint (which absorbs all wavelengths except those
in the red region of the spectrum) is mixed with a green paint (which absorbs all
wavelengths except those in the green region of the spectrum), the resulting red
+ green mixture will absorb all regions of the spectrum; it will therefore appear
black.
TEACHING NOTES
Most teachers of young children do work on colour naming and discrimination,
and on shades of colour and tints. This should include work on camouflage and
contrast, both of which can be used in the environment for safety purposes.
COLOUR MIXING
The primary colours for pigments are red, yellow and blue. The primary colours
for light are red, green and blue. Mixing colours of light is an additive process;
mixing- colours of paints is a subtractive process. This difference might be
explained to top juniors, but is too complicated for younger children. This does
not mean that children will not benefit from experimenting with mixing paints
and playing with colour filters and describing what they discover. An
explanation of the additive and subtractive processes of colour mixing is given
in Physics for GCSE (Swift, 1988).
SPECTRA
If practical work involving triangular prisms and spectra is done, get the children
to draw what they see. If a beam of light has been used children will often draw
the emergent band of colours as an arc, linking with their experience of
rainbows. Question them about this when you are actually viewing the effect.
This is another instance where the 'straight line' effect of light can be observed.
SCIENCE FOR PRIMARY TEACHERS
For younger children it is probably more satisfactory to place the prism so that
natural sunlight can pass through it to form a spectrum on the wall or ceiling.
The link with sunlight can then be made by shading the prism from the sun,
when the spectrum will disappear.
KEY POINTS
The diffraction phenomena observed with double slits and diffraction
gratings can be explained in terms o f constructive and destructive
superposition of waves transmitted by each slit.
For a double slit or diffraction grating, constructive superposition (bright
fringes) occurs when d sin 8, = nil.
Colour is determined by the wavelengths present in light.
5 WHAT KIND OF WAVE?
Main attainment targets and levels addressed in Section 5: none
STUDY NOTES
THE SPEED OF LIGHT
The speed of 3 x 1 0 S ms-I is so fast that it is beyond most people's
comprehension. We are not usually aware that light and other electromagnetic
waves take a finite time to reach us from their source. It is only when we think
in terms of astronomical distances that the transit times become appreciable. For
example, when communicating with astronauts on the Moon, radio waves took
about 1.3 seconds to travel from NASA Mission Control to the Moon, and the
astronauts' message took 1.3 seconds to return. The Moon is approximately
3.84 x 10% from Earth, so
distance - 3.84 x l o 8 m
= 1.3 s
3 x lo8m s-I
speed
If you have listened to recordings of those historic missions, you may have
noticed the gaps of several seconds that punctuate the conversations.
time =
On even larger distance scales, the Sun is approximately 1.5 x 10I1m from
Earth, so light from the Sun takes about 500 seconds (more than 8 minutes) to
reach Earth,
distance - 1.5 x 10" m = 500
- 3 x l o 8m S-I
speed
and light from distant galaxies may take millions or billions of years to travel
the vast distance to our Solar System.
time =
In Section 2 you saw that the speed of light depends on the material through
which it is travelling. Light travels fastest in a vacuum, and it travels only
slightly slower in air. However, in water it travels 1.33 times slower than in air
(v = 2.3 x IOsm s-I), and in glass it travels about 1.5 times slower than in
air (v = 2 x 10% ss-l).
ELECTROMAGNETIC WAVES
It is rather difficult to understand the concept of an electromagnetic wave. When
an electromagnetic wave passes a point, the associated magnetic field increases
and decreases sinusoidally, reversing its direction every half period. Its associated
electric field, at right angles to the magnetic field, increases and decreases in step
with the magnetic field. In the radio wave region of the spectrum, these
STUDY COMMENTARY FOR UNIT 10
oscillating magnetic and electric fields generate electric currents and voltages in
aerials connected to radio and TV sets.
TEACHING NOTES
There are no Teaching notes for this Section.
KEY POINTS
Light is just one part of the spectrum of electromagnetic radiation, which
ranges from short wavelength X-rays to long wavelength radio waves.
All electromagnetic waves travel in a vacuum at the speed of light
(3 x 108ms-I), and all involve similar variations of electric and magnetic
fields.
6 LIGHT AS PARTICLES
I Main attainment target and levels addressed in Section 6: none
STUDY NOTES
The message of this Section is that when light interacts with matter it behaves
as though it were made up of discrete particles, each of which carries energy and
momentum. You should read the complete Section to get the gist of the
arguments, but if you are short of time don't worry about understanding all of
the details of the photoelectric effect and the Compton effect.
TEACHING NOTES
There are no Teaching notes for this Section.
KEY POINTS
The photoelectric and Compton effects can be explained in terms of
photons, which are particles of electromagnetic radiation.
For radiation with frequencyf, the energy E of a photon and the magnitude
p of its momentum are given by the equations E = hf and p = hflc.
7 LIVING WITH THE WAVEPARTICLE DUALITY
See Section 8.
8 TV NOTES: LIGHT-IN
OF A MODEL
SEARCH
1 Main attainment target and levels addressed in Sections 7 and 8: none
1
STUDY NOTES
Sections 7 and 8 bring together the wave and particle models, emphasize their
complementary nature and explain when each should be used.
21
SCIENCE FOR PRIMARY TEACHERS
TEACHING NOTES
There are no Teaching notes for these Sections.
KEY POINTS
Two models are used to allow us to visualize the behaviour of light. The
wave model helps us to describe the propagation of light, and the particle
model is used to describe the interaction of light with matter.
Using these two models helps us to understand the practical applications of
light, for example solar cells and lens testing.
.
SUPPLEMENT: SOUND
Main attainment target and levels addressed in this Section: AT14: levels 1
t o 5
STUDY NOTES
S102 A Science Foundation Course mentions sound only in passing, so this
brief Supplement has been prepared to fill the gap. For more detailed coverage,
you should refer to the books cited in the Resources Section.
SOUND WAVES
Sound is a pressure wave, like the seismic P-waves that you studied in Units
5-6. Indeed, you can think of P-waves as sound waves--on a rather large scaletravelling through the Earth. Figure 11 shows the two representations of a sound
wave: pressure versus distance at a fixed time, and pressure versus time at a fixed
position. The wave illustrated is a pure middle-C tone travelling in air. The
graphs in Figure 11 correspond to the two graphs for a water wave in Frame 9 of
the AV sequence in Unit 10.
pressure
pressure
I
I
wave
(b)
FIGURE 1 1 Pressure versus distance (a) and pressure versus time (b) for a pure
middle-C tone in air. The first graph (a) indicates that the wavelength h is 1.25 m and
the second graph (b) indicates that the period T is 3.8 ms, so the frequency f (= 1/T)
is 260Hz.
SOUND CANNOT TRAVEL IN A VACUUM
Pressure waves need a medium through which to travel; they cannot travel
through a vacuum because pressure waves cannot occur in a vacuum. Sound
differs from light in this respect, because light (and other electromagnetic
radiation) can travel through a vacuum-light from the Sun and stars reaches
Earth after traversing vast distances through a very good vacuum.
STUDY COMMENTARY FOR UNIT 10
THE SPEED OF SOUND IN AIR
Sound travels at about 330m s-' (about 700mph) in air; this speed is about one
million times slower than the speed of light in air. This accounts for the delay
in hearing the thunder associated with a lightning flash: the light arrives almost
instantaneously (a delay of only 3 microseconds per kilometre distance), but the
sound of the thunder takes 3 seconds to travel a kilometre.
THE SPEED OF SOUND IN SOLIDS AND LIQUIDS
Sound travels faster in solids and liquids than in air. For example, in steel the
speed of sound is 5 130 m s-I, and in water it is 1 500 m s-I. Remember from
Units 5-6 that the speed of P-waves, and therefore of sound, is given by the
expression
where ly is the axial modulus (a measure of how incompressible a material is)
and p is the density of the material. Thus sound waves travel fastest in low
density, incompressible materials.
PRODUCING SOUND
FIGURE 12 This sequence of five diagrams covers one cycle of vibration of the
plate on the left. The shaded regions are places where the pressure is higher-they are
produced when the plate moves from left to right, compressing the air in front of it.
These high-pressure regions advance a distance of one wavelength h in a time equal to
the period of the vibration. (Note that similar pressure variations are generated on the
left of the plate, but these have not been shown.)
To produce audible sounds, something has to vibrate. A vibrating object
periodically compresses the air in contact with it, and these periodic
compressions travel through the air as a sound wave (Figure 12). A plucked or
bowed string of a musical instrument vibrates with a frequency that depends on
its length, tension and mass, and the frequency of the sound produced is the
frequency at which the string vibrates; altering any of these variables will cause
a change of frequency. Similarly, a struck object-whether a drum or a deskwill vibrate and thus produce sound. When a wind instrument is blown, pressure
waves are set up in the 'pipe', and the wavelength of these waves is directly
23
SCIENCE FOR PRIMARYTEACHERS
related to the distance between the mouthpiece and the open holes or valves, or
to the total length of the pipe if there are no holes. The longer this distance, the
longer the wavelength will be and therefore the lower the frequency of the sound
will be (Figure 13). Remember, v
so f = VIA, where v is the speed, f is
the frequency and A is the wavelength.
=fa
FIGURE 13 A treble recorder. The frequency above a hole is the frequency of the
sound obtained when that hole and all holes further down the instrument are
uncovered, and all holes closer to the mouthpiece are covered.
HEARING SOUND
We hear sound when pressure waves in the air enter our ears. These pressure
waves travel down the ear canal and force the eardrum to vibrate, and this in turn
causes small bones (ossicles) in the middle ear to vibrate (Figure 14). These
vibrations re transmitted to the cochlea in the inner ear, where they are sensed
by the- ewes that are connected to the auditory region of the brain.
4
I
ear canal
FIGURE 14 The human ear.
THE AUDIBLE FREQUENCY RANGE
The human sense of hearing extends from about 20Hz (i.e. 20 vibrations per
second) up to about 20 000 Hz. The maximum frequency detectable generally
decreases with age. Many animals can hear frequencies well above 20 000 Hz.
Sound with much higher frequencies, known as ultrasound, is used to produce
images of the inside of the human body, and in particular to monitor the growth
of the human foetus.
SOUND INTENSITY
The harder we strike an object (or blow down a pipe, or pluck a string), the
larger its amplitude of vibration, and the larger the amplitude of the pressure
waves that it produces, i.e. the louder the sound. The intensities of sounds are
generally expressed in units of decibels (dB); examples of, and effects of, various
sound intensities on this scale are shown in Table 2.
STUDY COMMENTARY FOR UNIT 10
TABLE 2 Various intensities of sounds in decibels (dB)
Sound
eardrum ruptures
jet engine
pain threshold
disco music
heavy street traffic
normal conversation
whisper
normal breathing
threshold of hearing
Intensity levelldB
120
60
20
10
4
Note that the decibel scale is a logarithmic scale: this means that for each
increase of 10 dB on the scale, the intensity of the sound increases by a factor of
10. Thus a 30 dB sound is 10 times more intense than a 20 dB sound, because
the difference of intensity is (30 dB-20 dB) = 10 dB. Similarly, an 80 dB sound
is 10' times more intense than a 30dB sound because the difference of 50dB, or
5 x 10 dB, is equivalent to 10 x 10 x 10 x 10 x 10-a factor of lo5. This
logarithmic scale agrees approximately with our subjective impression of the
intensity of sounds, since we perceive the difference between 70 dB and 80 dB
sounds to be the same as the difference between 80 dB and 90 dB sounds.
THE FREQUENCY OF MUSICAL NOTES
Figure 15 shows the frequencies of notes between middle-C and the C above
middle-C. This frequency range (i.e. C to C, or E to the next E, etc.) is known
as an octave, and an octave corresponds to an increase (or decrease) by a factor of
2 in frequency. Thus the A above middle-C has a frequency of 440Hz (Figure
15); the A one octave higher has frequency 880 Hz, and the A one octave lower
has frequency 220 Hz.
FIGURE 15 Frequencies of notes for one octave above middle-C.
THE DOPPLER EFFECT
The apparent frequency of a sound is modified when the source and listener are
moving relative to one another. This is known as the Doppler effect, and it will
be familiar from hearing police sirens passing by. When you move towards a
sound source, you pass successive wave-fronts more frequently than if you were
to just stand still and wait for them to reach you (Figure 16 overleaf). More
wave-fronts per second means that the frequency appears to be higher when you
approach the source. Conversely, when you move away from the source, fewer
wave-fronts will pass you per second than if you were stationary, and so the
frequency will appear to be lower. Similar effects occur if the source of the
sound is moving and the listener is stationary.
SCIENCE FOR PRIMARY TEACHERS
FIGURE 16 When moving towards the siren the driver passes more wave-fronts
per second than when travelling away from the siren. The apparent frequency
therefore decreases when the driver passes the siren.
TEACHING NOTES
There is a good cross-curricular link here. Children can explore and discuss in
what ways sounds are obtained from the school's musical instruments, e.g.
banging, scraping, shaking, plucking and blowing. Vocalizing and making
sounds with things other than musical instruments, and listening to sounds in
the environment, can also be included.
Some young children do not realize that we use our ears to hear with, and this
often has to be demonstrated to them. You could point out that the human head
contains three of the body's most important detectors. An effective way of doing
this is to select a child and to ask the other children to identify these three
detectors: the eyes detect light, the ears detect sound and the nose is sensitive to
smell.
It is important to stress that both the human eye and the human ear are capable
of detecting only a limited range of frequencies. Whereas the eye detects
electromagnetic waves with frequencies between 430 thousand billion Hz and
750 thousand billion Hz, the ear detects sound waves with frequencies between
20 Hz and 20 000 Hz. If we want to monitor electromagnetic waves and sound
waves outside these ranges, we must use other detectors.
SOUND TRAVELLING
Again, as with light, the concept of sound travelling (levels 2, 3 and 4) is
difficult for children. Sound is not tangible in the children's experience. It may
be useful to discuss the speeds of things that children know and recognize as
'travelling'. What is the fastest thing that they know about? With young
children even this may be complicated as they often confuse 'faster' with
'further'. Once the idea that light and sound travel is grasped then the link can be
made between seeing lightning and hearing thunder-light travels faster than
sound.
SOUND AND VIBRATION
Showing that sounds are produced by vibrating objects may require an
explanation of the word 'vibrating'.
If you put tiny tissue-paper riders on a string of a horizontal stringed instrument
and then play it, the paper will jump about until the sound stops. Touching
your throat lightly and humming is a way to feel the movement associated with
sound. Holding a vibrating tuning fork near the surface of water will make
ripples on the water. Twanging an elastic band stretched over a rigid container
will link movement with sound.
You can also show that sound can make things move by putting a few grains of
rice in a foil container and placing the container on the loudspeaker of a tape
recorder. Use a tape with a variety of types of music on it. What happens if you
STUDY COMMENTARY FOR UNIT 10
turn up the volume? Does the same thing happen with, say, the theme music
from 'Dr Who' and Beethoven's 'Pastoral Symphony'?
The idea of sounds travelling through different materials (level 3) can be
approached in several ways.
What sounds can we hear in the classroom? What can we hear from outside the
classroom? How can we hear these sounds? What happens if someone in another
room bangs on a radiator pipe? Can you hear a loudly ticking clock through the
wood of a table? Can you hear the tick of a clock through other materials? What
is the best sort of material to make a cover for the clock so that you can't hear
the tick?
EXPLORING SOUND
The programme of study for key stage 2 requires the children to explore changes
in pitch, loudness and timbre of a sound. Real musical instruments are of value
here, although you need to supervise closely so that expensive instruments are
not damaged by enthusiastic experimenters. As an introduction you could take
the front off an upright piano and look at which strings produce high notes and
which produce low notes. What happens when each pedal is pressed in turn?
How do you make a note louder or softer? If you could persuade friendly
musicians to bring instruments to school the children might be able to see and
hear the different sounds produced by long, short, thick and thin strings and by
altering the tension in the strings. In wind instruments the relationship between
different sounds and the length of the vibrating column of air produced by
blowing are more audible than visible, although children might like to try
'playing' different tubes (such as hosepipe, plastic tubing of different lengths and
diameters, or drinking straws) by blowing into them.
The children could make instruments with rigid boxes and 'strings' of various
thicknesses and tightness. They could make 'drums' using junk materials and
different skins and mallets. Timbre refers to the quality of sound-using different
materials to construct home-made instruments is probably the best way of
investigating this.
Level 5, with its 'understand' statements, again implies being able to use
knowledge in a new, given situation. This suggests using the knowledge for
problem-solving.
KEY POINTS
Sound is a pressure wave and so needs a medium through which to travelunlike light waves, which can travel through a vacuum.
In air, under ordinary conditions, sound travels at only about one-millionth
of the speed of light.
Most human beings can hear sounds with frequencies between 20Hz and
20 000 Hz; outside this range the sound waves are inaudible.
The intensities of sounds are usually expressed in decibels (dB).
Playing musical instruments provides cross-curricular links between music
and the physical sciences. Exploring the sound produced by different
instruments can help children to understand the physics of sound.
The apparent frequency of a sound is modified when the source and listener
are moving relative to one another (the Doppler effect).
SCIENCE FOR PRIMARY TEACHERS
RESOURCES
Department of Education and Science and the Welsh Office (1988) Science for
ages 5 to 16, HMSO.
Hunn, K., Smart, M. and Orme, N. (1989) Lightworks, Teacher's Pack,
available from Science Projects, Turnham Green Terrace Mews, Chiswick,
London W4 1QU.
Larnbert, A. (1985) Physics for First Examinations, Blackie.
Swift, D. (1988) Physics for GCSE, Basil Blackwell.
The following titles are from the Science 5/13 series, published by Macdonald
Educational for the Schools Council.
Collis, M. (1974) Early Explorations; and Tackling Problems, Part 2.
James, A. (1973) Like and Unlike, Stages 1 , 2 and 3.
Parker, S. (1972) Working with Wood,,Stages 1 and 2.
Parker, S. (1973) Coloured Things, Stages 1 and 2
Radford, D. (1973) Change, Stage 3; and Metals, Stages 1 , 2 and background.
Radford, D. (1974) Science, Models and Toys, Stage 3.
Richards, R. (1972) Early Experiences. and Time, Stages 1 and 2.
Richards, R. (1973) Holes, Gaps and Cavities, Stages 1 and 2; and Ourselves,
Stages 1 and 2.
QUESTIONS
These questions are designed to test your knowledge and understanding of the
Unit material. You should do them after studying the Unit and then check your
answers with your tutor.
Q1 Radioactive nuclei emit electromagnetic radiation whose photons carry
energy of the order 10-l2J. In what region of the electromagnetic spectrum is
this radiation? Select the one correct option from the list below.
A
y-rays
E
infrared
B
X-rays
F
microwaves
C
ultraviolet
G
radio waves
D
visible
Q 2 Figure 17 shows a 'snapshot' of two plane waves of the same
wavelength crossing each other. Each wave has an amplitude of 5 mm constant
over the area of overlap. The straight lines show the crests (maxima) of the
waves.
m-
wave 1
FIGURE 17 For use with 4 2 and 4 3 .
STUDY COMMENTARY FOR UNIT 10
What is the displacement at point C in Figure 17? Select the one correct option
from the list below.
A
Omm
E
-10mm
B
+5mm
F
between -5 and -10 mm
C
-5mm
G
between -5 and 0 mm
D
+10mm
H between +5 and +10 mm
4 3 Identify the point of maximum destructive interference in Figure 17.
Select the one correct option from the list below.
A
point A
E
point E
B
point B
F
point F
C
point C
G
halfway between points E and F
D
point D
4 4 A beam of light with a single wavelength 600nm is shone on a single
slit of width 4 x 10-6m and a diffraction image is observed on a screen fixed
15 cm behind the slit. How wide is the central diffraction maximum? Select
from the list below the one option that is closest to your answer.
A
0.015cm
E
2.3cm
B
0.15cm
F
4.6cm
C
0.85cm
G
23cm
Q5 Several statements about properties of electromagnetic radiation and
about the scientific models explaining these properties are given below. Select
the two statements that are correct.
A
In a vacuum, radio waves travel faster than light
B
Radiation of wavelength 104m can be detected by most human beings with
the naked eye
C
In the Compton effect, the scattered photon loses energy and the energy loss
depends on the angle at which the photon is scattered
D In the Compton effect, the momentum of the scattered photon is equal to
the momentum of the incident photon
E
In the photoelectric effect, photons are completely absorbed and their energy
transferred to the electrons
F
A simple wave model can explain all empirical observations of the
behaviour of electromagnetic radiation
G
Radiation of wavelengths below
H
The work function of a metal determines the maximum energy a photon can
have, if it is to release a photoelectron
m does not exhibit diffraction effects.
-
SCIENCE FOR PRIMARY TEACHERS
Q6 Figure 18 shows an experimental graph of maximum kinetic energy of
photoelectrons (E,),,, plotted against the frequency of radiation. (The 'scale for
(E,),,, is deliberately omitted.)
Select from the list overleaf (a) the one correct option for the value of the work
function of the material; and (b) the one correct option for the maximum E, for
radiation of 4 x lOI5 Hz.
f/1015Hz
FIGURE 18 For use with 4 6
NOTES