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Optometrists often measure
the power of lenses in dioptres
(D). The higher the number of
dioptres, the stronger the lens.
Convex lenses have positive
powers and concave lenses have
negative powers. Fairly weak
reading glasses would have a
power of about 1.0 D, whereas
a convex lens with a power of
+5.0 D is quite strong. A concave
lens of the same focal length
would have a power of –5.0 D.
A prescription contains a
number of figures for the optical
technician, which include the
overall power of the lenses
required, and information referring
to whether the wearer of the
glasses suffers from an irregularly
shaped eyeball, known as
astigmatism. If you wear glasses,
see if you can find out the power
of your lenses next time you visit
the optometrist.
S C I E N C E
see that the image is upside down. If we hold a convex lens close to an
object, it acts like a magnifying glass, producing an upright and larger
image. This is a different type of image from that which we produced
on the paper. We can’t reproduce this type of image on a screen, and it
is called a virtual image.
convex or converging lens
focus
focal length
Figure 4 . 2 1
A lens that is thicker in the centre is
called a convex or converging lens.
Strictly speaking, because the lenses
shown here have two surfaces that
curve outwards, they are called
bi-convex lenses. For simplicity, we
will refer to these as convex lenses.
Can you work out what a planoconvex lens would look like?
Figure 4 . 2 2
Parallel rays come to a focus after passing
through a convex lens.
work
EX P E R I M E N T
Investigating convex lenses
Aim
To investigate the different type of images that can be produced by a convex lens.
Materials
• ray box
• plastic convex lenses
4 .4
!
• power supply
• sheet of paper
• glass convex lens or a magnifying glass
Method
Before you start this activity, create a POE chart for the types of images you think you will
produce. Fill in the prediction column before each task and the observation column when you
complete each of the following three activities.
1. (a)Hold a glass convex lens or magnifying glass
in one hand and a sheet of white paper in the
other. Face a window in your room and move
the lens back and forth in front of the paper
until you produce an image of a distant object
on the paper, as shown in Figure 4.23. The
image may be easier to see if you look at the
window
back of the paper.
Figure 4 . 2 3
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Heinemann Science Link s 3
convex lens
white paper
(b)Sketch what you see. Is the image upright or inverted? Can you see any colours?
(c)Ask your partner to measure the distance from the lens to the paper when the image is in
focus. Record this as the focal length of the lens.
2. (a)Connect the ray box to the power supply. Place the multiple slit in the ray box to produce
parallel rays and place the thinner plastic convex lens in front of these rays.
(b)Observe and then trace the path the rays take as they pass through the lens.
(c) Repeat with the other convex lens. Which has the longer focal length?
light source
(bulb or candle)
convex lens
white paper
Figure 4 . 2 4
3. (a)Turn the light box around and open the mirrored doors so that the globe is visible
1. Does a convex lens invert images (turn them upside down)? Explain.
2. Does a convex lens split the white light into a spectrum of coloured light? Explain.
3. Why did the plastic lens and glass lens have different focal lengths?
4. What can you describe about the relationship between the focal length of a convex lens and its
04
(d)Record as much as you can about the nature of this image.
of paper.
thickness?
5. Try to fill in the explanation column of your POE chart for each of the three tasks.
Evaluation
1. Were you happy with the results that you found? Were there any errors?
2. Were there any conditions in your laboratory that made completing this experiment difficult?
3. Can you suggest any changes to improve the technique that you used?
chapter
Discussion
light effects
(or use a candle).
(b)Hold a glass convex lens between the bulb and a sheet of paper as shown in Figure 4.24.
(c)Move the lens backwards and forwards to try to obtain an image of the globe on the sheet
Concave lenses
We’ve seen that convex lenses can be used in a variety of ways to
produce different types of images. Use a concave lens to try to obtain
an image of your laboratory windows. What do you find? Concave
lenses produce images that are always upright, smaller in size and
virtual in nature. These are called diverging lenses because they spread
out parallel rays of light.
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concave or diverging lens
focus
focal length
Figure 4 . 2 5
A concave lens curves inwards on
both sides, giving it a thick rim around
the edge. Again, the correct term for
the lenses shown here is a bi-concave
lens, but we will refer to them simply
as concave lenses.
S C I E N C E
Figure 4 . 2 6
What is happening here? A concave
lens spreads light. It is more difficult to
measure the focal length of a concave
lens than a convex lens. To do this, we
need to trace the path of the rays leaving
the lens, to the single point where they
appear to have come from. This point is
the focus of the concave lens.
work
Investigating concave lenses
Aim
EX P E R I M E N T
To examine what happens to light when it passes through a concave lens and to measure
the focal length of such a lens.
!
Materials
• ray box with multiple slits
• concave lenses
• ruler
• power supply
• white paper
• pencil
Method
Before you start this activity, create a POE chart for how the focal length of a concave lens varies
with its thickness. Fill in the prediction column before each task and the observation column when
you complete each task.
1. Connect the ray box to the power supply. Place the multiple slit in the ray box so that several
parallel rays of light are produced.
2. Carefully draw around the outside of one of the lenses. Allow the rays to strike the lens.
Observe what happens to the rays as they pass through the lens and emerge at the other side.
3. Mark the path of the rays, remove the lens, and connect up the marks so that the paths of the
rays can be clearly seen.
4. Repeat the experiment using a concave lens of different thickness.
5. For each lens, trace back to where each set of rays appears to have come from and measure
the focal length.
Discussion
1. Fill in the explanation column of your POE chart.
2. Draw a T-chart showing two similarities and two differences between convex
and concave lenses.
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4 .5
Seeing is believing
The most important lenses to us are the lenses in our eyes. These bend
incoming light to focus at a point on our retina. Messages are sent
from here along the optic nerve to the brain. Our brain interprets this
information as an image and we are able to recognise and make sense
of our world. Look at the parts of the eye shown in Figure 4.28.
Figure 4 . 2 7
If our eyes focused like a magnifying glass, we could have a few issues!
Lens
A flexible structure
that enables light
to be focused on
the retina.
Ciliary body
The lens is connected to
the ciliary body. Muscles
in the ciliary body change
the shape of the lens.
Retina
The cells on the
retina called cone
and rod cells
absorb light rays
and turn them
into electrical
signals.
Optic nerve
Carries the electrical
signals from the
retina to the brain.
Fovea centralis
Cone cells are
more concentrated
in the fovea centralis
— the ares of sharpest
vision. Rod cells are
located on the retina.
Conjunctiva
A membrane that
covers the sclera.
The choroid has a
rich blood supply and
nourishes the retina.
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chapter
Cornea
Bends light into the
lens. Lies in front
of the iris. Is clear.
Eye muscles
Enable the eye to
rotate in its socket
light effects
Vitreous humour
Clear fluid between
the lens and retina.
The aqueous humour lies
between the cornea and the lens.
It is a clear, watery fluid produced
by the ciliary body to lubricate the
lens and cornea.
Sclera
White of the eye.
Dilator muscle
Enlarges the pupil,
allowing more light
into the eye.
Iris
Sphincter muscle
Makes the pupil smaller,
which stops light entering
the eye. Both muscles
enable the eye to focus.
Pupil
A round opening
that regulates the
amount of light that
enters the eye.
Figure 4 . 2 8
The structure of the eye.
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Getting into focus
To focus an image with a magnifying glass, we need to move the lens
backwards and forwards. Can you imagine if our own eyes focused in
this way? Instead, our eyes focus by the lens changing shape. For most
vision, the lens appears quite flat, but when we look at a close object,
the lens bulges in the centre in order to get a clear picture. This change
of shape of the lens in order to focus is called accommodation.
S C I E N C E
work
A c t i v i t y 4 .6
Looking for accommodation?
•Try to read this book from a long distance away. Bring the book closer.
When is it comfortable to read? Measure this distance from your eyes in centimetres.
How does this vary among your classmates and your teacher?
• Keep bringing the book closer. When do you lose focus altogether?
•Have a partner hold a pencil or pen in front of you and slowly bring it nearer. Try to keep focused.
What do you notice happens to your eyes?
Vision problems
scif ile
Imagine being able to alter the
lenses in your glasses without a
visit to the optometrist! E-Vision
of Virginia, USA, has developed
glasses with lenses that are filled
with liquid crystals and have a clear
chemical coating. A small voltage
from a battery stuck to the frame
arms changes the nature of the
lenses. The owner’s prescription is
stored in a memory chip attached
to the glasses.
Our eyes are amazing optical instruments, but, unfortunately, they
don’t always work perfectly. A person with short sight (myopia) might
be able to read a book easily, but have trouble reading a whiteboard
from the back of a classroom. This happens when the eyeball is too
long. The rays of light from a distant object are brought into focus in
front of the retina. Which type of lens do you think would be able to
correct this problem? Think back to the properties of a concave lens
and study Figure 4.29 to see how a concave lens can bring these rays
into focus at the retina.
rays from
distant object
focus in
front of
retina
rays from
close object
focus
behind
retina
Figure 4 . 2 9
You could draw an outline of an eye on white paper, and using a ray box with slits,
model the vision of a person with myopia, or short sightedness. Using a concave lens,
can you make this person’s vision become clear?
A person who has long sight (hypermetropia) can see distant
objects clearly, but has trouble focusing on close objects. This person
will need glasses for reading or when doing an activity that requires
them to concentrate on something close by. The problem is caused by
the eyeball being too short. Once again, it can be corrected with a lens,
but in this case, a convex lens is needed to bring the rays into focus on
the retina.
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concave
lens
convex
lens
focus on
retina
rays seem to
come from closer
focus on
retina
rays seem to come
from further away
Figure 4 . 3 0
Figure 4 . 3 1
Bertie the extendable robot hits middle age.
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scif ile
Records show that people have
been wearing glasses for more than
700 years! The first glasses were
worn by long-sighted Chinese and
Europeans. Short-sighted people
had to wait another 200 years before
suitable spectacles were produced
for them. In fact, Pope Leo X was
painted in 1517 while using a
concave lens for short sightedness.
Benjamin Franklin invented bifocal
glasses in 1784—all the better for
some active kite flying!
Figure 4 . 3 2
Ask some of the people in your family
who wear glasses if you can have a
look at them. Can your work out which
type of lenses their glasses contain?
chapter
As we get older, the lens in the eye loses flexibility and may not be
able to change shape as easily to focus on close objects. You may notice
this problem in people who read a book or newspaper held at arm’s
length! This problem is called lack of accommodation (presbyopia),
and is remedied by using a convex lens.
Because a person can have more than one vision complaint, they
might need one pair of glasses for reading, one for watching television
and another for walking around! To save the need for many pairs of
glasses, bifocal or trifocal glasses are prescribed. These have lenses that
are of varying strength, according to where light passes through them.
They are strongest at the lower edge and it is this part that is used for
reading. Lenses of graded focal length gradually change strength from
the bottom to the top.
Of course, if glasses are not for you, then you can consider using
contact lenses or the possibility of laser surgery. A contact lens is a
small lens that is worn directly on the cornea of the eye. These are
convex or concave, depending upon the problem to be corrected.
They are made from ‘hard’ plastic or ‘soft’ water-absorbing materials.
Because the lenses are in continual contact with the surface of the eye,
they must be kept very clean and be sterilised regularly. Alternatively,
disposable contact lenses are also commonly used.
Some people prefer to undergo laser surgery to correct a vision
problem. In this process, the surface of the cornea is altered to enable
the person to better focus light.
light effects
A person who is long sighted will need to use a convex lens to bring the rays into focus
at the retina.
Figure 4 . 3 3
Many people prefer contact lenses to
glasses because they have a wider, clearer
field of vision and they are convenient to
wear while playing sport.
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e
c
n
e
i
sc
in action
blinding trachoma
443P
Trachoma is the world’s leading cause of preventable blindness.
It is caused by the bacterium Chlamydia trachomatis and
infection is spread through contact with secretions from the
eye, nose or throat of an infected person.
The bacterium grows within the cells of the inner
eyelid. Repeated infections cause scarring and
eventually the eyelashes turn inwards and rub on the
cornea. This scars the cornea, which becomes opaque,
and results in poor vision and blindness.
Trachoma has existed since ancient times. It was
brought to Australia by the early settlers, and in
combination with the hot, dry climate, inadequate
housing and abundance of flies, it spread rapidly
from this time. By the 1900s, improved housing,
hygiene and more adequate sewerage facilities had
largely eradicated trachoma in developed nations
across the world. Father Frank Flynn, a trained
ophthalmologist who became a Catholic priest, was
the first to identify repeated trachoma infection
amongst Aborigines in the Northern Territory and
devoted his life to their treatment.
Figure 4 . 3 4
Trachoma is also called ‘sandy blight’ because it produces a
feeling of sand in the eyes.
Morocco
Mauritania
Senegal
Nepal
Niger
Vietnam
Sudan
Ghana
Ethiopia
Tanzania
areas where trachoma is endemic
Figure 4 . 3 5
Regions of the world where trachoma is endemic, being prevalent at the rate of 5–20% of the
population. Currently, 5.5 million people are blind or at a high risk of blindness from trachoma,
and a further 150 million are in need of treatment. The World Health Organization aims to
eradicate the disease worldwide by 2020.
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Figure 4 . 3 6
An estimated 20 000–30 000 Aboriginal children are afflicted with a disease
of the eye that had been eradicated from most societies by the early 1900s.
light effects
However, Dr Hugh Taylor, an expert on the disease who also
worked with Fred Hollows, has expressed his devastation that
in some communities, such as those in the Western District
and the Musgrave Ranges, the rates of childhood trachoma are
unchanged from 20 years ago. To prevent children currently
infected from becoming blind by the time they turn 40 or 50,
they need to wash their faces twice a day, have access to an
antibiotic treatment once or twice a year and have adequate
living conditions.
04
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In the 1970s, Fred Hollows raised the awareness of the problem
and was instrumental in the establishment of the National
Trachoma and Eye Health Program. From 1976 until 1978,
under this program, more than 465 Aboriginal settlements
were visited, some 62 000 Aboriginal people were examined,
1000 operations were performed, 27 000 people were treated for
trachoma and guidelines were set up to lead to the eradication
of the disease. Currently, the incidence of trachoma in many
towns and larger communities has virtually disappeared.
blinding trAchoma
questions
Discuss
Recently, the Federal Government introduced the concept of a ‘shared responsibility agreement’,
between the government and indigenous communities. Such an agreement with the Mulan Aboriginal
Community in the Kimberley region of Western Australia was established in late 2004. The Federal
Government agreed to contribute $172 000 for the installation of fuel bowsers at Mulan on the condition
that members of the community abide by certain guidelines, such as washing children’s faces twice
a day and ensuring rubbish bins are emptied twice a week. Not surprisingly, this policy has been
controversial, with some saying that it is unfair to deny services that other Australians are entitled to
without abiding by an agreement. Opponents of the scheme have argued that it does not show respect
to the indigenous community and that people involved should be part of the decision-making process.
1. Do you agree or disagree with the approach taken by the Federal Government? Use the
issues report template from Chapter 11 Skills link to present a report on this issue.
2. If you disagree, then work with a team of other students to research this issue further and
write a set of guidelines that could be followed to improve this situation.
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questions 4. 2
1. Working with a partner, draw up a list of as many
devices that you can think of that use lenses.
2. Choose the correct alternative and complete
each statement.
(a)A convex/concave lens is also called a
converging lens because it brings parallel
rays of light together to a point.
(b)A convex/concave lens is called a diverging
lens because it spreads out parallel rays of
light.
(c)If an image can be obtained on a screen, it
is called a real/virtual image.
(d)The image formed when we look through a
magnifying glass is a real/virtual image.
(e)A person who is short sighted can see
close/distant objects clearly, but has
difficulty focusing on close/distant objects.
(f)A convex/concave lens can be used to
correct the vision of a long-sighted person.
3. Copy Figure 4.37 and complete the paths that
the rays of light will take when they pass through
these lenses. Name the lens types shown.
(a)Decide on the way that you will present your
information, the important points that you
need to include, including how this vision
can be corrected, and the steps that are
taken in testing and treating these problems.
(b)Together, negotiate a role for each member
of your group and develop a timeline for
tasks.
(c)After completing this task complete
the group evaluation table.
9. Table 4.1 lists typical focal lengths for different
optical devices.
(a)Which object uses a lens with the shortest
focal length? Why do you think this is so?
Table 4 .1
Object
Spectacles
Focal length
(m)
1
Camera lens
0.05
Microscope objective lens
0.004
(b) Which lens is the strongest of the three?
(c)Sketch what you think each of the lenses
could look like.
Figure 4 . 3 7
4. Which type of lens only produces smaller,
upright images?
5. (a)Name the type of lens that produces a real
image.
(b) How could you test if an image is real?
(c)Must the object be close to or further away
from the lens in order for this to occur?
Explain your answer.
6. Con asks Narim to pass him a convex lens of
focal length 25 cm from a box containing many
lenses of different focal lengths. How could
Narim find the correct lens?
7 . Natasha can see writing on the board clearly but
has difficulty reading from a book. State the type
of eye problem that Natasha has and the type of
lens that could help her.
8. Imagine that you and a small group of
classmates have been employed by the state
government to produce new information
packages for primary school children to inform
them of things that can go wrong with their
vision.
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10. In practice, there is a limit to the amount
of magnification we can achieve with a
single lens. For this reason, many devices
use a combination of lenses. Research how a
compound microscope, overhead projector,
telescope, binoculars or a modern camera
makes use of multiple lenses. Present your
findings in a format of your choice.
11. The Fred Hollows Foundation works with
people in Nepal, Eritrea, Vietnam, north
Queensland and Torres Strait to provide
affordable treatment for cataract blindness.
Find out more about the work and philosophy
of the Foundation.
(a)Create a poster or newspaper or magazine
advertisement about the work of the
foundation.
(b)You could organise a social service activity
such as a casual clothes day to raise money
for the Fred Hollows Foundation or some
similar organisation.
4.3 Colour my world!
scif ile
Newton believed that numbers had
mystical meanings related to the
laws of the universe. Even though it
is almost impossible to distinguish
the colour indigo in the spectrum,
Newton included it to give a total of
seven colours, matching the seven
notes of the musical scale, the seven
seas, the seven days of the week,
and the seven openings in our head.
Clearly, the number seven was
regarded as pretty special!
light effects
We enjoy the spectacle of colour every time we see a rainbow or gaze at
a sunset. Most of the light that enables us to see travels as white light
from the Sun. In 1666, Isaac Newton passed a narrow beam of light
through a glass prism and discovered that this light was split into a
brilliant band of colours. Newton realised that white light is actually a
mixture of all of these colours of the spectrum. Can you think of times
when you’ve seen light split into such colours before? The splitting of
white light into the colours that make it up is called dispersion.
Each colour of light that we see has a slightly different frequency
(and wavelength) from other colours. As a result, each individual
colour is bent, or refracted, a slightly different amount when entering
a transparent material, such as glass. Red light always bends the least,
and violet light the most, so that white light is split into its component
colours.
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04
Figure 4 . 3 8
This prism has split a beam of white light into a spectrum of colour. It is difficult to see
distinct boundaries between colours, but Newton stated that there are seven that make
up the full spectrum: red, orange, yellow, green, blue, indigo and violet. You might recall
from your work in Heinemann Science Links 2, that the spectrum of colours that makes
up visible light is just one small component of the entire band of electromagnetic
radiation that makes up the complete electromagnetic spectrum.
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Why do we see colours?
white light
red light
apple
Figure 4 . 3 9
A red apple reflects red light and
absorbs other colours.
scif ile
We can use a device called a
spectrometer to analyse light to
see the colours that make it up. By
examining light from distant stars,
astronomers can estimate their age
and determine their composition.
After a traffic accident, forensic
scientists can analyse the light
emitted from burning a chip of
paint found at the scene to find
what type of paint it is. This can
give clues to the age, model and
type of car it came from. Everything
produces its own characteristic
spectrum when burnt, much like an
identifying fingerprint.
Figure 4 .4 0
These overlapping structures, each measuring only one-fifteenth of a millimetre in size,
are scales from the wing of a peacock butterfly. The fringes of colour are produced
from white light being refracted by the tiny ridges on the scales.
How is it that we see one type of apple as red and another as green?
Pigments on the surface of an object give it colour. An object is seen
as red if it reflects red light towards our eyes and absorbs orange,
yellow, green, blue and violet light. In reality, the apple may reflect a
little orange light as well, but this just affects the shade of red that we
see. In the same way, a blue yo-yo reflects blue light (and probably a
little green and violet) and absorbs all other colours of light. Knowing
this, can you work out how a white sheet of paper looks white? Because
white light is made up of all of the colours of light added together,
white objects reflect all of the light that hits them. Black objects absorb
all colours, reflecting none.
A quick colour change
We know that white light can be produced by shining all colours
together. Surprisingly, we can also make white light from just three
colours of the spectrum—red, green and blue. For this reason, these
are called the primary colours of the spectrum. If we combine these
primary colours in pairs, we then produce three secondary colours:
red + green = yellow
red + blue = magenta
green + blue = cyan
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Shining red, green and blue light together produces white light.
Our eyes have three special types of cell called cones to detect
colour. Each type of cone is sensitive to one of the primary colours.
Combinations of signals from these three types of cell give us our full
colour view of the world.
Figure 4 .4 2
A colour television builds its images using thousands of tiny dots called pixels. Electron
beams are directed to illuminate a phosphor that is red, green or blue in colour. Mixing
the light produced from these three colours produces all of the other colours we see on
the TV. Combining coloured lights in this way is called additive colour mixing.
When coloured ink pigments are
added together, such as in the
printing of a book, the combined
colour becomes darker as more
sections of the visible spectrum are
absorbed rather than reflected. This
type of colour combination is called
subtractive colour mixing. The three
subtractive primary colours are cyan,
magenta and yellow. Black ink is
also used in this process because
the black produced from primary
colours is not usually intense enough
to help create the image that is
desired. A magenta print will reflect
red and blue light, absorbing green
light. If magenta paint is mixed
with cyan paint, a blue colour is
produced. Can you explain this by
looking at the colours shown in the
photo? Can you predict how green
and red colours are mixed using this
technique?
light effects
Figure 4 .4 1
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#
Coloured filters are used widely in photography and the theatre to
provide a range of lighting effects. A filter is a transparent material that
will transmit only certain colours. A filter works by absorbing specific
parts of the spectrum and transmitting others, so a green filter placed
in front of a globe will allow only green light through. Most filters
actually transmit a little of the colour either side of their own in the
spectrum as well. One spectacular aspect of a live theatrical production
is the dramatic lighting. Floodlights and spotlights can be varied
creatively to produce colours and effects to enhance the production.
105