Intermediate 2
ROR
Waves and Optics
page 1
Intermediate 2
Waves and Optics
Speed of Sound
One of the earliest forms of communication was to use
sound, talking to each other. We are going to measure
the speed of sound in air.
Apparatus:
microphones
Timer
The speed of sound apparatus will be set up for you.
1. Copy the table on page 4 into your jotter.
2. Make sure the microphones are reset.
3. Hit the bottle sharply.
4. Record the time into your table.
5. Repeat until the table is completed.
6. Calculate the values for the final column.
Remember speed = distance ÷ time
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page 2
Intermediate 2
Waves and Optics
Speed of Sound Cont.
Reminder ms = x10-3s.
Attempt
1
2
3
4
Dist. Between time (ms)
microphones (m)
1.5
1.5
1.5
1.5
Average speed
speed (m/s)
Copy and complete
When the sound passes the first microphone the timer
________ . When the sound reaches the second
microphone the timer stops. The speed is calculated by
dividing the distance between the microphones by the
time on the timer.
The average speed of sound in this experiment was
found to be ___________ m/s.
Now complete 3.1 – 3.8 from the problems booklet.
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page 3
Intermediate 2
Waves and Optics
Waves
With the increase in mobile technology
most of our day to day communications are
completed using waves passing through the
air, wires or fibre optics.
We need to
understand how to describe a
wave so that we are able to
talk about waves in a
meaningful way.
Collect a waves diagram and stick it into your
jotter.
Listen to your teacher describe the parts of the wave.

A

A

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page 4
Intermediate 2
Waves and Optics
Copy
Wavelength (symbol  units metres, m).
It is the distance between two successive points on a
wave in phase.
Amplitude (symbol A, units metres, m)
It is measured from the centre line to the crest or
trough. It is a measure of how much energy a wave
carries.
Frequency (symbol f, unit hertz, Hz)
This is how many waves are produced each second. This
is the same as the number of waves that pass a point in
one second.
Period (symbol T unit second s)
This is the time to produce one wave.
Speed (symbol v, unit metres per second, m/s)
This is the distance a wave travels in one second.
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Intermediate 2
Waves and Optics
The wave equation
We can find the speed of a wave by measuring how far
the wave travels in a known time interval. [speed of sound
experiment]
It is also possible to calculate the speed of a wave if we
know the wavelength and frequency of the wave.
Speed = frequency x wavelength
v = f x 

Example: A water wave has a wavelength of 50cm.
Twenty waves pass a point in 10 seconds.
Calculate the speed of the wave.
v=?
f = 20 waves in 10s = 20/10 waves in 1s = 2Hz
 = 50cm = 0.5 m
v = f x  = 2 x 0.5 = 1m/s
Now complete 3.9 – 3.16 from the problems booklet.
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page 6
Intermediate 2
Waves and Optics
Radio and TV
On December 12, 1901, Marconi
made the first transatlantic
wireless transmission between
Poldhu in Cornwall (U.K.) and St.
John's, Newfoundland, by using
Morse code. In 1906, Reginald
Fessenden (U.S.A.) constructed a
high-frequency alternator and
succeeded in transmitting the
human voice over the radio.
Signals have been transmitted by radio for over 100
years.
Radio and Television signals travel through the air at a
speed of 300 000 000 m/s.
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Intermediate 2
Waves and Optics
Longitudinal and Transverse waves
Watch the demonstration by your teacher who will show
the difference between longitudinal and transverse
waves.
Collect a transverse and longitudinal wave diagram.
Copy
In longitudinal waves the vibration of the particles is in
the same direction as the direction of energy transport.
In transverse waves the vibration of the particles is at
right angles to the direction of energy transport.
Sound is a longitudinal wave.
Light, water waves and waves on a string are transverse
waves.
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page 8
Intermediate 2
Waves and Optics
Electromagnetic Spectrum
Light waves travel at 300 000 000 m/s. Visible light is a
small part of a family of waves called the
electromagnetic [EM] spectrum. The spectrum is
illustrated below.
microwaves
TV
FM
It is important that you know the order of the waves in
the EM spectrum.
From the long wavelength end the waves are
Radio, Microwaves, Infrared, Visible, Ultraviolet, X-rays,
Gamma.
Collect a spectrum diagram. Make up a mnemonic to help
remember the order of the spectrum.
Now complete 3.17 from the problems booklet.
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page 9
AM
Intermediate 2
Waves and Optics
Waves Checklist
Collect a checklist sheet and traffic light it.
Work in groups to
change any red or
amber entries to
green. If no one in the
group can get green
then see your teacher
for assistance.
When you have successfully changed all
your lights to green you should attempt
questions 3.18 – 3.22 from the problems
booklet. Try to use your notes as little as
possible.
Mark your answers using the mark
scheme. Any questions that you get
wrong you need to find out why and
write a short note to explain your
mistake.
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Intermediate 2
Waves and Optics
Reflection
wooden block
normal
mirror
light ray
θ
incident
ray angle
ray box
Set up the apparatus as shown
on a piece of blank paper. Mark
on the normal line at right angles
to the mirror. Shine a single
light ray at the mirror so that it
strikes the normal. Mark the
direction of the incident and
reflected light rays on the
paper.
Repeat this for three other
angles. Collect a reflection
diagram for your jotter
Measure the angles then copy
and complete the table below.
Investigate what happens when you shine the ray along
the path taken by the reflected ray.
Table of results
incident ray angle (°)
reflected ray angle (°)
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page 11
Intermediate 2
Waves and Optics
Reflection (cont.)
Answer the following questions in sentences
1. How does the incident angle compare with the
reflected angle?
2. What happens to the path of the ray when its
direction is reversed?
3. Copy and complete the paragraph below
When light is reflected, the angle is always measured
between the light ray and the _________. The angle
of incidence is _________to the angle of reflection.
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page 12
Intermediate 2
Waves and Optics
Curved reflectors
Watch the teacher’s
demonstration of curved
reflectors.
Set up the apparatus shown
below
+7D lens
ray box
curved
reflector
Draw a neat diagram to show what happens when the
rays of light strike the curved reflector.
Switch off the ray box but leave the apparatus in
place.
Answer the following questions in sentences.
1. Where is the light brightest in your experiment?
2. Can signals other than light be reflected from a
curved reflector? Explain your answer.
3. Where would be the best position for a receiver in a
curved reflector?
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Intermediate 2
Waves and Optics
Curved reflectors(cont.)
Switch on the ray box again. Mark the
focus point on your diagram. Remove the
ray box. Be very careful not to disturb
the paper or curved reflector. Connect
the 6V lamp to the battery pack and
place it at the focus point.
Draw a diagram to show what happens to the light from
the lamp.
Copy the following.
When a signal strikes a curved reflector it is reflected
to a focus point, increasing the strength of the signal
at that point. If a signal is produced at the focus point
it will be reflected out in a narrow beam. This allows it
to be aimed in a particular direction.
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page 14
Intermediate 2
Waves and Optics
Total Internal Reflection
We are now going to investigate what happens when a
light ray strikes the inside surface of a piece of glass.
θi
Set up the apparatus as
Perspex block shown on the left.
Gradually increase the size
of angle θi. Watch carefully
to see what happens to the
ray after it strikes the
inside surface.
light from ray box
Change the value of angle θi until the ray just reflects
inside the Perspex block. Try to get the ray to pass along
the straight edge of the block. Measure the angle θi.
When the ray leaving the block travels along the edge of
the block angle θi is called the critical angle. If θi is
increased above this value, total internal reflection takes
place.
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Intermediate 2
Waves and Optics
Answer the following questions in sentences
1. How does the size of the angle of the ray leaving
the block compare with θi?
2. What is the value of the critical angle for the
Perspex block?
3. Does the ray always leave the block through the
straight edge?
4. When the ray does not leave through the straight
edge, what does it do?
Copy the diagram of the optical fibre below
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page 16
Intermediate 2
Waves and Optics
Optical Fibres
Today’s telephone system has many features that did not
exist twenty years ago.
I
Physics
 Caller display
 Ring back
 Last call 1471
 Answer phone service 1571
In order for these
services to work
extra information has
to be carried on the
telephone line. Metal
wires can not carry
enough information.
Thin glass fibres
were developed to carry light signals. These fibres can
carry enough information to run the services.
Your teacher will show you a demonstration of an optical
fibre in use.
Collect and complete an optical fibre worksheet.
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page 17
Intermediate 2
Waves and Optics
Refraction
Copy:
Refraction is when light changes its speed going from one
substance into another. This may also result in a change
of direction for the light.
Collect a ray box, a perspex block and a power supply.
Set up the apparatus as shown by your teacher.
normal
angle of incidence
Carefully mark the path of the ray of light through the
block.
Copy the diagram into your jotter showing accurately the
path of the light through the block.
Mark on the angle of refraction onto your diagram.
Copy the passage choosing the correct word.
When light passes from air into glass the ray of light
bends towards/away from the normal.
When light passes from glass into air the ray of light
bends towards/away from the normal.
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page 18
Intermediate 2
Waves and Optics
Lenses
Lenses make use of the effect of refraction that causes
light to change direction.
In this experiment you will examine the effect of
different lenses on rays of light.
Collect a ray box, a power supply, a lens activity sheet
and a selection of lenses.
Concave
Convex
Investigate what happens to the light rays as they pass
through the lenses. Use the activity sheet to draw the
result of the rays passing through the lenses.
Copy and complete:
When light rays pass through a convex lens they come
together/move apart. When light rays pass through a
concave lens they come together/move apart. The more
curved a lens is the greater/less the effect on the light
rays. Rays of light that spread out before they reach the
lens represent light from a nearby/far away object.
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Intermediate 2
Waves and Optics
Focal Length
Previously you found out that different shapes of lens
affect light in different ways. A way of identifying the
effect of a lens is to measure its focal length. Lenses
are normally identified by their lens power.
The focal length is the distance from the lens to a clear
image of a distant object. You will now measure the focal
length of a selection of lenses. You will investigate the
relationship between the focal length and lens power.
Copy the table below
lens power
(dioptres)
+20
+10
+6.6
+5
+3.8
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focal length
(metres)
page 20
1
lens power
Intermediate 2
Waves and Optics
Measuring Focal Length Instructions.
Stand across the lab from the window.
Move the lens back and forwards towards the wall until a
clear sharp image of the trees at the back of the school
are obtained.
Collect a lens power/focal length table.
Measure the distance between the lens and wall.
Enter your results and complete the table.
Answer the following questions
1. How does the image compare with the original object?
2. What happens to the focal length as the lens power
increases?
3. How do the final two columns in the table compare?
4. Write down an equation that we could obtain from the
results of this experiment.
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page 21
Intermediate 2
Waves and Optics
Copy
The focal length of a lens should be given in metres. The
power of a lens is measured in a unit called the dioptre
[die-opt-er]. It has the abbreviation D. If a lens power is
positive [ +3D] it is a converging lens. If a lens power is
negative [ -8D] it is a diverging lens.
Calculating Focal Length
Example: The power of a lens is + 10D. Calculate its focal
length.
1
1
f=?
f= =
= 0.1 m
P
10
P = +10D
A convex lens is found to have a focal length of 25cm.
What is the power of this lens?
f = 25cm = 0.25 m
1
1
P= =
= +4D
P=?
f 0.25
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page 22
Intermediate 2
Waves and Optics
Ray diagrams
In this activity you will learn how to draw a ray diagram
to show how a converging lens forms the image of an
object.
You already know from the previous activity that an
image of an object can be obtained on a screen.
There are millions of light rays coming from any object,
fortunately we can simplify our diagrams by drawing just
two.
Collect a laptop and complete the ray diagrams Power
point exercise.
Now complete 3.27 – 3.31 from the problems booklet.
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page 23
Intermediate 2
Waves and Optics
Image on the retina
The diagrams below show how light from a near object
and light from a distant object produce an image on the
retina.
near object
distant object
Copy the diagrams into your jotter.
Copy and complete the following passage:
When light from a near object enters the eye the lens
becomes more/less curved to focus it onto the retina.
When light from a distant object enters the eye the lens
becomes more/less curved to focus it onto the retina.
Muscles in the eye allow the lens to change shape.
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page 24
Intermediate 2
Waves and Optics
Long and short sight
When someone suffers from long sight they can only see
distant objects clearly. Near objects appear blurred.
The lens and cornea combine to make the rays of light
focus ‘long’ of the retina.
This defect can be remedied using corrective lenses.
A convex lens will cause the rays to come together and
focus on the retina.
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Intermediate 2
Waves and Optics
Long and short sight (cont.)
When someone suffers from short sight they can only
see near objects clearly. Distant objects appear blurred.
The lens and cornea combine to make the rays of light
focus ‘short’ of the retina.
This defect can be remedied using corrective lenses.
A concave lens will cause the rays to spread out and
focus on the retina.
Colect and complete the ‘Corrective lenses’ sheet. Have
your teacher check it then stick it into your jotter.
Now complete 3.32 – 3.33from the problems booklet.
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page 26
Intermediate 2
Waves and Optics
Reflection and Refraction Checklist
Collect a checklist sheet and traffic light it.
Work in groups to
change any red or
amber entries to
green. If no one in the
group can get green
then see your teacher
for assistance.
When you have successfully changed all
your lights to green you should attempt
questions 3.34 – 3.39 from the problems
booklet. Try to use your notes as little as
possible.
Mark your answers using the mark
scheme. Any questions that you get
wrong you need to find out why and
write a short note to explain your
mistake.
ROR
page 27