Transcript PowerPoint Presentation - Int2 unit3

In unit 4 we will learn about
Waves and Optics.
Waves are everywhere in
nature – sound waves, visible
light waves, earthquakes,
water waves, microwaves,
Mexican waves…
Understanding optics is
important in use of lenses – for
sight, cameras, entertainment
and many other applications.
Waves & Optics
What do you know?
About waves?
About optics?
Key words: waves, energy transfer, transverse,
longitudinal, frequency, speed, wavelength, amplitude.
By the end of this lesson you will be able to:
State that a wave transfers energy.
Use the following terms correctly in context: wave,
frequency, wavelength, speed, amplitude,
period.
State the difference between a longitudinal wave
and a transverse wave, and give an example of each.
Waves & Energy Transfer
Waves transfer energy.
The energy transferred by waves can be
considerable!
Indian Ocean 2004
Indonesia 2005
Waves transfer energy…
Waves transfer energy…
Properties of Waves
What is meant by the axis of a wave?
The axis is the line running
through the middle of the
wave pattern.
What is meant by the crest of
the wave?
The crest is the top part
of the wave
…and the trough?
The trough is the bottom
part of the wave.
What is the amplitude of the wave?
The amplitude is the
distance from the axis to
the crest
or from axis to trough.
Definition of Wavelength?
The wavelength is the
distance after which the
wave pattern repeats itself
– the distance between two
identical points on the wave
Wavelength is given the
symbol
λ
pronounced
lambda.
http://www.teachersdomain.org/resources/lsps07/
sci/phys/energy/wavelength/assets/lsps07_int_wa
velength/lsps07_int_wavelength_swf.html
Frequency
The frequency of the wave is the number
of waves each second.
It is measured in hertz (Hz) which just
means “per second”.
http://www.teachersdomain.org/resources/lsps07/sci/phys/energy/freque
ncy/assets/lsps07_int_frequency/lsps07_int_frequency_swf.html
http://www.m2m.ecs.soton.ac.uk/Wcb886307c3e67.htm
Frequency
Calculate the frequency of each of the
following waves:
125 waves passing a point in 10 seconds.
16 waves passing a point in 24 seconds.
30 waves passing a point in 1 minute.
Period
The period of a wave is the time taken
for one complete wave to pass a point.
It is measured in seconds (s).
Frequency & Period
The link between the frequency and
period of a wave
1
f 
T
Frequency & Period
Rearranging
1
T
f
Longitudinal Waves
In this type of wave,
particles vibrate back and
forwards along the direction
the wave is travelling.
Longitudinal Waves
A sound wave is a longitudinal
wave.
http://www.acoustics.salford.ac.uk/schools/index1.htm
Transverse Waves
In this type of wave,
particles vibrate at right
angles to the direction of
motion of the wave.
Transverse Waves
Light, and all forms of
electromagnetic radiation, are
transverse waves.
http://www.acoustics.salford.ac.uk/schools/index1.htm
Homework for next lesson
Learn:
transverse and longitudinal waves
properties of waves
-
Axis
Crest
Trough
Amplitude
Wavelength
Frequency
Period
Key words: electromagnetic spectrum,
wavelength, frequency
By the end of this lesson you will be able to:
State in order of wavelength, the members of the
electromagnetic spectrum: gamma rays, X-rays,
ultraviolet, visible light, infrared, microwaves ,
TV and radio.
State that radio and television signals are
transmitted through air at 300000000 m/s and that
light is also transmitted at this speed.
What do you know?
About waves?
About light?
About the electromagnetic
spectrum?
Myth or Reality?
Visible light is the
only type of light
Myth!
Visible light is a tiny slice of the radiation
that makes up the electromagnetic
spectrum.
What is radiation?
Myth or Reality?
All radiation is harmful.
Myth!
Not all radiation is harmful. It
depends on the dose.
Light is a form of radiation. All
parts of the electromagnetic
spectrum are considered radiation,
but only X-rays and gamma rays
are ionising radiation.
Myth!
Ionising radiation is dangerous because it can
penetrate body tissues and cause cell damage.
Ultraviolet light from the Sun causes sunburn,
which is a common form of “harmful” radiation.
Where does light come from?
What is a light wave?
Light is a disturbance of electric and
magnetic fields that travels in the form
of a wave.
Like all waves it can be described as
having peaks, troughs, frequency,
wavelength, amplitude and it transfers
energy.
What is the electromagnetic
spectrum?
The electromagnetic spectrum
consists of all the different
wavelengths of electromagnetic
radiation, including light, radio
waves, and X-rays.
All travel at
Radio Waves
The longest wavelength and lowest
frequency.
Radio waves are longer than 1 mm.
Radio wavelengths are found
everywhere: in the background
radiation of the universe, in
interstellar clouds, and in the cool
remnants of supernova explosions.
Radio Waves
Radio stations use radio wavelengths
(10 cm – 1000 m) of electromagnetic
radiation to send signals that our
radios then translate into sound.
These wavelengths are typically 1 m
long in the FM band.
Radio Waves
Radio stations transmit em radiation,
not sound. The radio station encodes
a pattern on the em radiation it
transmits, and then our radios
receive the em radiation, decode
the pattern and translate the
pattern into sound.
Microwaves 0.1 – 10 cm
Basis of almost all
space communications.
Infrared 0.1 cm to 0.00007 cm
Heat!
Infrared 0.1 cm to 0.00007 cm
http://www.teachersdomain.org/r
esources/ess05/sci/ess/earthsys
/irgallery/assets/ess05_int_irgal
lery/ess05_int_irgallery_swf.ht
ml
Infrared
wavelengths
Heat!
are about
same size as a
single
bacteria.
Visible Light
Visible light covers the range of
wavelengths from 400 to 700 nm.
Our eyes are sensitive only to this
small portion of the electromagnetic
spectrum.
Visible Light
The Sun emits most of its
radiation in the visible range, which our
eyes perceive as the colours of the
rainbow.
Ultraviolet
Ultraviolet radiation
has wavelengths of
10 to 310 nm (about the
size of a virus).
Young, hot stars
produce a lot of
ultraviolet light and
bathe interstellar
space with this
energetic light.
Ultraviolet
X-rays
X-rays range in
wavelength from 0.01 to
10 nm (about the size of
an atom).
They are generated, for
example, by super
heated gas from
exploding stars and
quasars, where
temperatures are near
a million to ten
million degrees.
Gamma rays
Gamma rays have the
shortest wavelengths,
of less than 0.01 nm
(about the size of an
atomic nucleus).
This is the highest
frequency and most
energetic region of the
em spectrum.
Gamma rays can result
from nuclear reactions
taking place in objects
such as pulsars, quasars,
and black holes.
Low frequency
Rabbits
Mambo
In
Very
Unusual
eXpensive
Gardens
High frequency
Long wavelength
Radio
Microwaves
Infrared
Visible Light
UV
X-rays
Gamma Rays
Short wavelength
All e-m waves travel at the speed of light!
Transverse Waves
In this type of wave,
particles vibrate at right
angles to the direction of
motion of the wave.
Transverse Waves
Light, and all forms of
electromagnetic radiation, are
transverse waves.
http://www.acoustics.salford.ac.uk/schools/index1.htm
What have you learned today?
Key words: speed of sound, distance, speed,
time
By the end of this lesson you will be able to:
Describe a method of measuring the
speed of sound in air, using the
relationship between distance, time
and speed.
describe how sound is produced
describe sound as a wave which transfers
energy, explaining what is meant by the
frequency of a sound
Key words: frequency, wavelength, speed,
amplitude, period
By the end of this lesson you will be able to:
Carry out calculations involving the relationship
between distance, time and speed in
problems on water waves, sound waves, radio
waves and light waves.
Carry out calculations involving the relationship
between speed, wavelength and frequency
for waves.
What do we know
about sound?
Sound
How does sound travel?
How fast does sound
travel?
Sound Vibrations
The aim of this activity is to show how sound energy is
produced.
Think!
How is sound produced by the tuning fork?
How does changing the length of the tuning fork affect
the sound produced?
Look at the frequencies of the tuning forks. As the
frequency increases, how does the pitch change?
Energy Transfer
The aim of this activity is to show how sound energy is
transferred.
Think!
Signal generator
Loudspeaker
What is happening to the candle flame?
What is moving the flame?
Can you explain how sound energy is transferred?
How fast does sound travel?
We can use sound switches and an
electronic timer to measure the speed of
sound.
What two things do we need to measure to
find the speed of sound?
• the distance travelled.
• the time taken.
How fast does sound travel?
Why is an electronic timer used when
measuring the speed of sound?
Measuring Average Speed
distance
speed 
time
Measure the time taken for sound to travel
from one microphone to the other.
Repeat your measurements to improve
reliability.
microphone
Electronic timer
metre stick
metal plate
hammer
Oscilloscope Patterns and
Frequency
Think!
How is sound produced by the loudspeaker?
At very low frequency, what do you observe?
As frequency increases, what happens to the pitch of
the note?
What is the lowest frequency you can hear? And the
highest?
Octaves and Frequency
Think!
Look at the signals produced for different instruments
and different notes.
As the pitch increases, what do you notice about the
number of waves on the screen? What is happening to
the frequency?
Look at two notes an octave apart. What happens to the
number of waves on the screen. What does this tell you
about frequency?
Oscilloscope Patterns and
Loudness
Think!
As the sound becomes louder, what do you expect to
happen to the oscilloscope trace?
Does loudness affect the frequency of the signal?
What can sound travel
through?
What do we need for sound to travel?
A solid, liquid or a gas. Sound can’t travel
through space because it is a vacuum.
What can sound travel
through?
When the air was pumped out of the
container….
it was no longer possible to hear the bell
ringing.
Virtual Int 1 Sound & Music -> Using Sound -> Vacuum
Longitudinal Waves
In this type of wave,
particles vibrate back and
forwards along the direction
the wave is travelling.
Longitudinal Waves
A sound wave is a longitudinal
wave.
http://www.acoustics.salford.ac.uk/schools/index1.htm
Loudness of Sound
We measure the loudness of sound in
decibels – dB.
Spending too long listening to loud sounds
can permanently damage your hearing.
http://www.tlc-direct.co.uk/Technical/Sounds/Decibles.htm
http://www.tlc-direct.co.uk/Technical/Sounds/decible4.swf
Hearing Damage in the Workplace
If you are regularly exposed to noise
at 80dB or above, your employer must
carry out risk assessment.
If 85dB or above, they must provide
protection.
http://www.hse.gov.uk/noise/demonstration.htm
Max recommended length of exposure
85 dB
8 hours
88 dB
4 hours
91 dB
2 hours
94 dB
1 hours
97 dB
30 minutes
100 dB
15 minutes
103 dB
7.75 minutes
106 dB
3.75 minutes
How big is 1dB sound file
http://www.phys.unsw.edu.au/jw/dBNoFlash.html
Date Accessed 27/03/2008
Nightclub at 106dB
4 minutes unprotected
2 hours protected
Gunshot 140dB
never unprotected
1.5 minutes protected
Is your music too loud?
“Preliminary data on iPods and similar devices have
found lower maximum levels - above 100 decibels (the
noise volume of a chainsaw; risk of hearing damage
after two hours), but not higher than 115 decibels (a
football game in a loud stadium; risk of hearing damage
after 15 minutes)…To fully understand the potential
impact of these devices, it is important to knowthat the
sound is travelling a tiny distance from your earbud to
your eardrum rather than being diffused in a football
stadium or concert arena.”
http://www.hearingconservation.org/docs/RealityCheckonMusicNIHL.pdf
Extract from article by Gregory Mott, Washington Post. Date Accessed 26/03/2008
Ultrasound
What is the normal range of human
hearing?
20 Hz – 20000 Hz (or 20 kHz).
What is ultrasound?
Ultrasound is sound above 20000 Hz.
Imaging with Ultrasound
Medical ultrasound used for imaging a
foetus is between 1 and 20 MHz
(10000000 and 20000000 Hz!)
How is ultrasound used for imaging?
http://www.layyous.com/ultasound/ultrasound_video.htm
Imaging with Ultrasound
http://www.mayoclinic.com/health/ultrasound/MM00084
A handheld transducer is held on the
stomach. This changes electrical energy
to sound energy which is transmitted in
waves into the body.
Imaging with Ultrasound
The waves are reflected off the boundaries
between materials which have different sound
properties (e.g. the amniotic fluid and the
foetus). The transducer also acts as a receiver
– detecting the echo and turning sound into an
electrical signal.
Imaging with Ultrasound
By recording the time taken for the echo
to be detected, and the intensities of the
echoes, and using the speed of sound in
tissue, a computer can build up a detailed
image of the foetus.
Imaging with Ultrasound
The skin is covered in jelly during the
ultrasound procedure. Why is this?
Good contact is important. Otherwise the
ultrasound waves will simply bounce off
the outer skin and no image will be
obtained. It makes movement of the
transducer easier. It ensures a clear
picture is obtained.
Imaging with Ultrasound
Why is ultrasound used instead of
X-rays?
Ultrasound causes no harmful effects to
the foetus. It is low power so can be used
safely to image the foetus.
Other Uses of Ultrasound
Doppler ultrasound is a newer technique
which can be used to measure blood flow.
Ultrasound can also be used to break up
kidney stones – faster and safer than
surgical removal.
Using Sonar
SOund NAvigation and Ranging
Used for underwater ranging –
first patented in 1913 –
prompted by Titanic disaster.
Using Sonar
How does Sonar work?
What is it used for?
Sonar Use
Radio Communication
What are radio (and TV) waves?
They are a type of electromagnetic radiation
that travel through air at
3 x 108 m/s.
Each radio station broadcasts with a
different frequency or
wavelength – remember the
speed is always the same.
The frequency of radio waves is much
higher than the frequency of sound
waves.
Imagine a wave where 2 waves pass a
point in 1 second.
number of wavesproduced
frequency
time taken in seconds
frequency f = 2 Hz.
amplitude
10
0.1
0.2
distance (m)
What is the length of the wave produced
in this time?
2 waves in 1 second -> 2 wavelengths.
2 wavelengths = 2 x 0.1 = 0.2 m
So two waves are produced in 1 s.
That is 0.2 m in 1 s.
This links distance and time which gives us…..
amplitude
speed
10
0.1
0.2
distance (m)
Calculating Wave Speed
There are two formulae which can be used
to calculate wave speed.
Speed is a distance in a given time.
distance
speed 
time
Calculating Wave Speed
speed = frequency x wavelength
speed in m/s
v = fλ
wavelength in
metres
frequency in Hz
How fast is a radio
or TV wave?
Radio and TV signals are part of the
electromagnetic spectrum.
This means they travel at the
light which is 3 x 108 m/s.
speed of
Key words: reflection, angle of incidence,
angle of reflection, light rays
By the end of this lesson you will be able to:
State that light can be reflected.
Use the terms angle of incidence, angle
of reflection and normal when a ray of
light is reflected from a plane mirror.
State the principle of reversibility of a
ray path.
Reflection of Light
Watch the demonstration with the laser optics kit.
What is meant by the normal?
What is meant by the angle of incidence?
What is meant by the angle of reflection?
What do you notice about the angles of incidence and
reflection? How do they compare?
Virtual Int 2 Physics -> Waves & Optics -> Reflection -> Reflection from a Plane Mirror
The normal, angle of incidence
and angle of reflection
Complete the sentences:
The normal is
The angle of incidence is
The angle of reflection is
Reflection of Light
The normal is at a right
angle (90º) to the
surface.
Law of Reflection
Θi
ΘR
angle of incidence = angle of reflection
Flash animation - reflection
Reversibility of Rays
Law of Reflection
N
R
ΘR
The normal is at a right
angle (90º) to the
surface.
I
Θi
angle of incidence = angle of reflection
Flash animation - reflection
What have you learned?
Key words: refraction, incidence, normal
By the end of this lesson you will be able to:
State what is meant by the refraction of
light.
Use the terms angle of incidence, angle
of refraction and normal.
Draw diagrams to show the change of
direction as light passes from air to glass
and glass to air.
Refraction
http://www.planet-scicast.com/view_clip.cfm?cit_id=2728
Rays of Light
In any material (air, glass, water) light will
travel in a straight line.
We represent light using
ray diagrams.
Drawing Light Rays
We represent light using
ray diagrams.
What two things must you remember
when drawing light rays?
Refraction of Light
We saw that light can be reflected from
a flat surface.
When light passes from one material to
another it changes speed. This change in
speed can cause it to change direction.
We call this
refraction.
Refraction of Light – Air to Glass
Watch the demonstration with the laser optics kit.
What is meant by the normal?
What is meant by the angle of incidence?
What is meant by the angle of
refraction?
What do you notice about the angles of incidence and
refraction? How do they compare?
Virtual Int 2 Physics -> Waves & Optics -> Refraction through blocks and prisms
Activity – Air to Glass
By the end of this practical session you
will know:
how to draw a normal
how to identify, and measure, angles of
incidence and refraction as light
passes from air to glass.
what is meant by
refraction.
Place the block on the outline provided
Shine the light ray along the line
Accurately mark the path of the refracted ray
Then switch off the light source and remove the block
Remove the block and draw in the refracted ray
Carefully draw in the normal
Mark the angles of incidence (i) and refraction (r)
Use a protractor to measure the angles of incidence
(i) and refraction (r)
Record your results in the space provided
i=
r=
Use a protractor to measure the angles of incidence
(i) and refraction (r)
Think!
What happens when the ray of light
shines into the block along the normal?
Is refraction taking place?
What is meant by the angle of
incidence?
Think!
What is meant by the angle of
refraction?
What happens when the angle of
incidence is greater than 0o?
Refraction: Air to Glass
What happens when the ray of light enters the block
along the normal?
There is no change in direction of the
light i.e. it goes straight through. The
light is refracted – it changes speed.
What do we mean by angle of incidence?
The angle of incidence is the angle
between the incoming ray and the normal.
Refraction: Air to Glass
What do we mean by the angle of refraction?
The angle of refraction is the angle
between the refracted ray and the
normal.
What happens when the angle of incidence is greater
than 0o?
When the angle of incidence is greater
than 0o the light changes direction due to
refraction.
refracted ray
angle of refraction r
angle of incidence i
normal
r is less than i
incident (or incoming)
ray
Refraction
Light travels in a straight line.
When light travels from one material (e.g. air) to
another (e.g. glass) it changes speed. This is called
refraction.
This change in speed sometimes causes it to change
direction.
Refraction is the change
in speed of light
as it passes from one medium to
another.
Refraction of Light – Glass to Air
Watch the demonstration with the laser optics kit.
What is meant by the normal?
What is meant by the angle of incidence?
What is meant by the angle of
refraction?
What do you notice about the angles of incidence and
refraction? How do they compare?
Virtual Int 2 Physics -> Waves & Optics -> Refraction through blocks and prisms
Activity – Glass to Air
By the end of this practical session you
will know:
how to draw a normal
how to identify, and measure, angles of
incidence and refraction as light
passes from air to glass.
what is meant by
refraction.
Place the block on the outline provided
Shine the light ray along the line
Accurately draw in the path of the refracted ray
Then switch off the light source and remove the block
Accurately draw in the path of the refracted ray
Then switch off the light source and remove the block
Carefully draw in the normal
Measure the ANGLE OF INCIDENCE, i
Measure the ANGLE OF REFRACTION, r
Record your results in the space provided
i=
r=
Refraction –
Glass to Air
Angle of
Incidence (i)
o
0
(along the normal)
15o
Ray Box
o
35
42o
o
75
Angle of
Refraction (r)
Think!
What happens when the ray of light
shines into the block along the normal?
Is refraction taking place?
What is meant by the angle of
incidence?
Think!
What is meant by the angle of
refraction?
What happens when the angle of
incidence is greater than 0o? What do
you notice as you increase the angle of
incidence?
Refraction – Glass to Air
What happens when the ray of light enters the block
along the normal?
There is no change in direction of the
light i.e. it goes straight through.
What do you notice as you increase the angle of
incidence?
The angle of refraction increases. Some
reflection can also be seen.
Refraction – Glass to Air
What is the relationship between the angle of incidence
and the angle of refraction?
The angle of refraction is always larger
than the angle of incidence.
What happens when the angle of incidence is 42o or
above?
Total internal reflection occurs and no
light escapes.
Blue light refracts
more than red light
Why does refraction occur?
Virtual Physics Animations -> Refraction of Waves
Why do we need to know about
refraction?
Refraction explains:
how the eye focuses light;
sight defects such as long
and short sight and how to
correct them using
contact lenses or glasses.
All lenses refract
light. Lenses are used
in cameras, telescopes
and binoculars.
Total Internal Reflection and the
Amazing Disappearing Coin
Key words: total internal reflection, critical
angle, optical fibres
By the end of this lesson you will be able to:
Explain, with the aid of a diagram, what is meant
by total internal reflection.
Explain, with the aid of a diagram, what is meant
by the “critical angle”.
Describe the principle of an optical fibre
transmission system.
Think!
What happens when light passes from one
material to another?
What happens when light passes from
glass to air? How does the angle of
incidence compare with the angle of
refraction?
Critical Angle
Flash animation
We’ve seen that as light passes from glass
to air, it changes direction away from the
normal.
When light passes from glass to air there
is an angle beyond which light cannot
escape from the glass.
This is called the critical
angle.
What happens at the critical
angle?
What happens at the critical angle?
At the critical angle, the angle
of refraction is 90°. Beyond the
critical angle, Total Internal
Reflection (TIR) occurs.
When TIR occurs the angle of
incidence = angle of reflection.
The normal, angle of incidence
and angle of reflection
Complete the sentences to give definitions of
the normal, and angles of incidence and
reflection.
The normal is
The angle of incidence is
The angle of reflection is
Reflection of Light
The normal is at a right
angle (90º) to the
surface.
Law of Reflection
Θi
ΘR
angle of incidence = angle of reflection
Flash animation - reflection
Reversibility of Rays
Law of Reflection
N
R
ΘR
The normal is at a right
angle (90º) to the
surface.
I
Θi
angle of incidence = angle of reflection
Flash animation - reflection
Total internal reflection is used in
fibre optics which are used in:
medicine ;
cable television ;
internet ;
telephone access.
Optical Fibres
Optical fibres are about the
thickness of a human hair.
Each fibre is a thin piece of glass,
coated with a thin layer (or
cladding) of another glass.
Using Optical Fibres in
Communication Systems
Optical fibre – sound transmitter and receiver.
What are the steps needed to use optical
fibres in a communication system?
For example in a telephone system?
Transmission
The sound signal must be changed into an
electrical signal. The signal is a series of
electrical pulses.
What input device is used to convert sound to
an electrical signal?
The electrical signal is converted into
pulses of light which are transmitted
through the optical fibre via total internal
reflection.
Receiving
The light signal is changed back into an
electrical signal using a photodiode.
The electrical signal is converted into
sound. What output device is used to
convert an electrical signal to sound?
What are the advantages of using
optical fibres in communications?
The signal that passes along the fibre is not
electrical so less likely to suffer from
interference.
Cheaper to make than copper.
Can carry far more information than copper
wires. (One optical fibre cable could take all the
telephone calls being made in the world at any
time!)
There is less loss of signal – so not as
many amplifiers required.
Disadvantage – can be more difficult to
join fibres than copper wires.
Using Fibre Optics in Medicine
When light is produced using a normal
electric light bulb, what energy
transformations take place?
Electric energy -> Light energy
Electric energy -> Heat energy
In an ordinary light bulb, the
filament heats to about
2500oC – the bulb is very hot!
You can use fibre
optics to provide a
“cold” light source
inside the body
that means the light
travels but it doesn’t
get hot!
Controls
End probe containing
coherent bundle,
incoherent bundle, lens
and surgical instruments
Eyepiece
ENDOSCOPE
Light injected here
This is an endoscope image of
the inside of the throat. The
arrows point to the vocal chords
A COHERENT BUNDLE: A bundle of
optical fibres in which the optical
fibres are neatly arranged relative to
one another. Such a bundle are used
for the transmission of images.
A NON-COHERENT FIBRE bundle, as
you would expect, does not have this
neat layout since they need only
transmit light for illumination
purposes. They are cheaper to
produce.
Endoscopes
An endoscope consists of two bundles of optical
fibres: the LIGHT GUIDE and the IMAGE GUIDE
The purpose of the LIGHT GUIDE is to send
light (but not heat!) INTO the patient
The IMAGE GUIDE brings light back, allowing the
doctor to see inside the body
Light travels along both of these bundles by
TOTAL INTERNAL REFLECTION
Key words: curved reflectors, received
signals, transmitted signals
By the end of this lesson you will be able to:
Explain the action of curved reflectors on
certain received signals.
Explain the action of curved reflectors on
certain transmitted signals.
Describe an application of curved
reflectors used in telecommunication.
Curved Reflectors
Watch the demonstration.
What do you notice about the light when a
curved reflector is used? Where do we
make use of curved reflectors?
Dish Aerials
A dish aerial can be used either
to receive or to transmit a radio
(or microwave) signal.
Virtual Int 1 Physics -> Telecommunications -> Satellites -> Curved reflectors
receiver
receiving dish aerial
The receiving dish aerial gathers in parallel
rays from a distant object and reflects the rays
to one point called the focus. The receiving
aerial is placed at the focus to receive the
strongest signal.
If we tilt the dish…
receiving dish aerial
transmitter
transmitting dish aerial
The transmitting aerial allows a strong
(concentrated) signal to be sent in a particular
direction from the transmitting dish aerial.
transmitting dish aerial
Dish Aerials
What is the advantage of using a dish
with a larger area?
The dish collects more of the incoming
beam and focuses it – resulting in a
stronger signal. Satellite TV in Scotland
requires a bigger dish than in England!
Dish Aerials
A dish aerial can be used either to
receive or to transmit a radio (or
microwave) signal.
Tasks & Homework for …
YPQ2 3.23, 3.28
McCormick & Baillie Int 2 Physics
p133 qu 2, p145-146 qu 1, 2
YPQ2 3.25, 3.35
McCormick & Baillie Int 2 Physics
p133 qu 1
What have you learned?
Key words: converging, diverging, lenses,
parallel light rays, power, focal length
By the end of this lesson you will be able to:
Describe the shape of converging and
diverging lenses.
Describe the effect of converging and
diverging lenses on parallel rays of light.
Carry out calculations involving the
relationship between
a lens.
power and focal length of
Lenses
Lenses refract light.
A convex lens is one which is thicker in
the middle than at its edge. It bulges out.
Lenses
When we shine parallel rays through a
convex lens…
normal
Convex lenses bring parallel light rays to a focus. They
are converging lenses.
Lenses
The focal length of the lens is the
distance between the centre of the lens
and the focus.
normal
Convex lenses bring parallel light rays to a focus. They
are converging lenses.
When parallel light rays enter a
CONVEX lens they are CONVERGED
(focused)
The rays meet at a point called the
PRINCIPAL FOCUS (‘F’ in the diagram)
The distance from the centre of the lens to the
principal focus is called the FOCAL LENGTH
f
(f)
Lenses
A concave lens is one which is
thinner in the middle than at its edge. It
“caves” in.
Lenses
When we shine parallel rays through a
concave lens…
normal
Concave lenses cause parallel light rays to diverge.
Measuring the Focal Length of a
Concave Lens
The focal length of a lens is the distance
from the centre of the lens to where the
rays come together.
But a concave lens causes the rays to diverge!
Diverging lenses have a negative focal length.
Focal length f
When light rays enter a CONCAVE lens
they are DIVERGED (spread)
This time the principal focus is
BEHIND the lens
As before, the distance to the principal
focus is the FOCAL LENGTH
f
Because the focal length is in the opposite
direction, we give it a NEGATIVE value
Summary (so far)
Type of
What it does
lens
Convex
Concave
Focal
length
CONVERGES
light (brings the Positive
rays together)
DIVERGES light
(spreads the
Negative
rays)
Thick and Thin Lenses
Observe the demonstration with the lenses.
Can you predict which lenses will be
converging? And which will be diverging?
Is there a relationship between the
thickness of the lens and the focal
length?
Measuring the Focal Length of a
Converging Lens
By the end of this practical session you
will be able to measure the focal length of
a converging lens.
Measuring the Focal Length of a
Converging Lens
Step 1: Identify a distant light source. Hold
the lens between the light source and screen
(piece of paper).
Step 2: Move the lens back and forward until a
focused image appears on the screen.
Step 3: Measure the distance between the
centre of the lens and the screen on which the
focused image appears – this is the focal length.
Measuring the Focal Length of a
Converging Lens
Note your results.
Why must a distant light source be used?
Which lens had the shortest focal length?
Which lens had the longest focal length?
Which lens is the weakest?
Which lens is the strongest?
Could you predict this without making
measurements?
Compare these 2 convex lenses:
The thicker lens has a shorter focal length
f
f
The thicker lens “bends” the light more, so we
say it has a greater POWER than the thin lens
f
f
Power of Lenses
A more powerful lens causes more refraction.
The power of a lens is measured in
power 
1
dioptres.
focal length
POWER OF A LENS
To calculate a lens’ power, use this equation:
power 
focal length
For short:
P 
DIOPTRES
(D)
1
1
f
METRES
(m)
Power of Lenses
Converging lenses have a positive focal
length and a
positive power.
Diverging lenses have a
length and a
negative focal
negative power.
Summary
Type of
lens
Convex
What it does
Focal
length
Power
CONVERGES
light (brings the
rays together)
Positive
Positive
DIVERGES light
Concave
(spreads the
Negative Negative
rays)
A lens has a focal length of 20 cm.
Find its power.
What do I know?
f = 20 cm = 0.2 m
1
1
P 
 5D
f 0.2
What type of lens is this? How do you know?
A lens has a power of -12D. What type of lens
is this?
Diverging / concave – since the power is
negative.
A concave lens has a power of -12D. Find its
focal length.
What do I know?
f=?
P = -12 D
1
1
f  
 0.083 m
P  12
Focal Length and Power
Questions
For each of the lenses below calculate the focal length or powe
and state whether the lens is convex (converging) or concave
(diverging).
Find the focal length of 1-9
1 Lens power
2 Lens power
3 Lens power
4 Lens power
5 Lens power
6 Lens power
7 Lens power
8 Lens power
9 Lens power
+10D
–10D
+100D
-24D
3D
16D
24D
-32D
-1.33D
Find the power of 10-18
10 Focal length 0.5 m
11 Focal length -50cm
12 Focal length 20cm
13 Focal length 0.2cm
14 Focal length 0.4cm
15 Focal length -0.25cm
16 Focal length -0.10cm
17 Focal length 30cm
18 Focal length -15cm
Which is the most powerful lens? Which has the longest focal length?
Key words: converging, diverging, lenses,
retina, long sight, short sight, ray diagram, image
formation, real, virtual, magnified, diminished, inverted
By the end of this lesson you will be able to:
Draw a ray diagram to show how a converging
lens forms the image of an object placed at
different distances in front of the lens.
Describe the focusing of light on the retina
of the eye.
State the meaning of long and short sight.
Explain the use of lenses to correct long and
short sight.
Creating Images with Lenses
Two important rules when dealing with
lenses.
1. If the light ray enters the centre of
the lens then is passes straight through.
Creating Images with Lenses
2. If the light ray enters the lens
travelling horizontally, it is refracted
through the focal point.
Source of light relatively distant
from the lens
F
Virtual Int 2 Physics – Waves and Optics
F
Source of light relatively distant
from the lens
F
F
We see a real image is formed. The image is smaller
than the object (it is diminished), and is inverted.
A real image is one which can be formed on a
screen.
Source of light more than two
focal lengths from the lens
F
F
F
We see a real image is formed. The image is smaller
than the object (it is diminished), and is inverted.
A real image is one which can be formed on a
screen.
Source of light between one and
two focal lengths from the lens
F
F
F
We see a real image is formed. The image is larger
than the object (it is magnified), and is inverted.
A real image is one which can be formed on a
screen.
Source of light between less than
one focal length from the lens
F
F
The rays do not meet.
F
We trace the rays back
to find where the rays
would meet.
F
F
F
F
F
The image is virtual. It is an
illusion of the eye and brain – but
you could not place a screen here
and see an image
F
upright and
magnified.
The image is
F
F
F
Source of light closer to the lens than its
focal length
The rays do not meet beyond the lens (the rays are diverging).
When the rays are projected back they do meet. Since light
does not actually pass through this point, it is a virtual image.
The human eye will bring the light to focus and the virtual
(magnified) image seen. This is the image we see with a
magnifying glass.
Ray Diagrams
Virtual Int 2 Physics -> Waves & Optics ->
Lenses -> Ray Diagrams & More Ray
Diagrams
Magnification of a Lens
An image formed by a lens is either
magnified (bigger) or
diminished (smaller).
length of image
magnification 
length of object
Summary
Objects more than two focal lengths:
The image is real, inverted and diminished.
Objects between one and two focal
lengths:
The image is real, inverted and magnified.
Object less than one focal length:
The image is virtual, upright and magnified.
The Human Eye
Light enters the front of the eye at the
cornea. It is transparent, and it is here
that most of the refraction occurs.
The Human Eye
4
1
2
3
1 controls the amount
of light which enters
4 the eye
1
It is called
2
the iris
3
1
2
4
2 is the hole in the
middle of the iris. It
allows light to enter
the eye.
It is called
the pupil
3
In bright light will the pupil be large or
small?
It will be smaller to prevent too much
light from getting in to the eye.
4
1
2
Along with the cornea,
3 focuses the light. It
is called
the jelly lens
3
How does the eye lens focus on
objects at different distances?
Muscles around the lens make it
thicker or thinner.
4
1
4 is where the “light
picture” is built up. It
is called
the retina
2
3
The retina contains nerve cells which
are affected by light and send signals to
the brain along the optic nerve.
4
1
4 is where the “light
picture” is built up. It
is called
the retina
2
3
To see objects clearly, light must be
focused on the retina.
Virtual Int 2 Physics -> Waves and Optics ->
Applications of Lenses -> The Eye
Remember!
Most of the
refraction occurs in
the cornea –
not the lens!
Common Sight Defects
Long and short sight
are caused by the failure of
the eye’s lens to bring the
light rays to a focus on the
retina.
Long and Short Sight
Observe the demonstration of long and short
sight.
Where do the rays meet when a person is long
sighted?
What type of lens can be used to correct this?
Where do the rays meet when a person is short
sighted?
What type of lens can be used to correct this?
Long Sight
A person with long sight can see far away
objects clearly. Objects close up appear
blurred.
Long sight occurs when the light rays are
focused “beyond” the retina.
Long Sight
Remember that rays do not actually pass beyond
the back of the eye!
Long sight can occur because the eyeball is
shorter than normal from front to back. It can
also be caused by the ciliary muscles not
Long Sight
Long sight can occur because the eyeball is
shorter than normal from front to back. It can
also be caused by the ciliary muscles not
relaxing to make the lens fat enough. Remember
the fatter the lens, the more powerful and the
more quickly rays are brought to a focus.
Long Sight
A person with long sight can see far away
objects clearly. The rays are parallel and can be
brought to a focus on the retina.
Objects close up appear blurred. The rays
coming from a close up object are diverging and
the eye does not bring them to a focus on the
retina.
Long Sight
A convex lens converges the light rays
before the cornea/lens and allows the eye
to focus the light on the retina rather
than behind the retina.
Short Sight
A person with short sight can see close
objects clearly. Objects far away appear
blurred.
Short sight occurs when the light rays are
focused in front the retina.
Short Sight
Short sight can occur because the lens in the
eye is very curved. It can also be caused by the
ciliary muscles not making the lens thin enough.
Short Sight
A person with short sight can see close up
objects clearly. The rays are diverging and can
be focused on the retina.
Distant objects appear blurred. The rays
coming from a distance object are parallel and
the eye brings them to a focus too soon.
Short Sight
A concave lens diverges the light rays
before the cornea/lens and allows the eye
to focus the light on the retina rather
than in front of the retina.