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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!
Waves transfer energy…
Waves transfer energy…
Properties of Waves
What is meant by the axis of a wave? The axis is the line
runningthrough the middle of thewave 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 bottompart of the wave.
What is the amplitude of the wave?
The amplitude is thedistance from the axis tothe crest
or from axis to trough.
Definition of Wavelength?The wavelength is the
distance after which thewave pattern repeats itself– the distance between twoidentical 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_wavelength/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/frequency/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 andperiod of a wave
Tf
1=
Frequency & Period
Rearranging
fT
1=
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 wavesproperties 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. Itdepends on the dose.
Light is a form of radiation. All
parts of the electromagneticspectrum are considered radiation,
but only X-rays and gamma raysare 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 andmagnetic fields that travels in the form
of a wave.
Like all waves it can be described ashaving peaks, troughs, frequency,wavelength, amplitude and it transfersenergy.
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 WavesThe longest wavelength and lowestfrequency.
Radio waves are longer than 1 mm.
Radio wavelengths are foundeverywhere: in the backgroundradiation of the universe, ininterstellar clouds, and in the cool
remnants of supernova explosions.
Radio Waves
Radio stations use radio wavelengths (10 cm – 1000 m) of electromagneticradiation to send signals that ourradios then translate into sound.
These wavelengths are typically 1 mlong in the FM band.
Radio Waves
Radio stations transmit em radiation,
not sound. The radio station encodes
a pattern on the em radiation ittransmits, and then our radiosreceive the em radiation, decodethe pattern and translate thepattern 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
Heat!
Infrared wavelengths are about same size as a single bacteria.
http://www.teachersdomain.org/resources/ess05/sci/ess/earthsys/irgallery/assets/ess05_int_irgallery/ess05_int_irgallery_swf.html
Visible Light
Visible light covers the range ofwavelengths 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 itsradiation in the visible range, which our
eyes perceive as the colours of therainbow.
Ultraviolet
Ultraviolet radiationhas wavelengths of10 to 310 nm (about the
size of a virus).
Young, hot starsproduce a lot ofultraviolet light andbathe interstellarspace with thisenergetic light.
Ultraviolet
X-raysX-rays range inwavelength from 0.01 to
10 nm (about the size of
an atom).
They are generated, for
example, by superheated gas fromexploding stars andquasars, wheretemperatures are neara million to tenmillion degrees.
Gamma raysGamma rays have theshortest wavelengths,of less than 0.01 nm (about the size of an atomic nucleus).
This is the highestfrequency and mostenergetic region of the
em spectrum.Gamma rays can resultfrom nuclear reactionstaking place in objects
such as pulsars, quasars,
and black holes.
RabbitsMamboIn Very UnusualeXpensive
Gardens
RadioMicrowavesInfraredVisible LightUVX-raysGamma Rays
Low frequency
High frequency
Long wavelength
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,
timeBy the end of this lesson you will be able to:
Describe a method of measuring the
speed of sound in air, using therelationship between distance, timeand speed. describe how sound is produceddescribe sound as a wave which transfers
energy, explaining what is meant by thefrequency 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 inproblems on water waves, sound waves, radiowaves and light waves.Carry out calculations involving the relationship
between speed, wavelength and frequencyfor waves.
What do we know about sound?
Sound
How does sound travel?
How fast does soundtravel?
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?
Sound Vibrations
The aim of this activity is to show how sound energy is transferred.
Think!
What is happening to the candle flame? What is moving the flame?
Can you explain how sound energy is transferred?
Energy Transfer
Signal generator Loudspeaker
How fast does sound travel?
We can use sound switches and anelectronic timer to measure the speed ofsound.
What two things do we need to measure tofind 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
Measure the time taken for sound to travel from one microphone to the other.Repeat your measurements to improve reliability.
timedistance
speed=
Electronic timer
microphone
metal plate
metre stick
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 thecontainer….
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 indecibels – dB.
Spending too long listening to loud sounds
can permanently damage your hearing.
http://www.tlc-direct.co.uk/Technical/Sounds/Decibles.htmhttp://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 hours88 dB 4 hours91 dB 2 hours94 dB 1 hours97 dB 30 minutes100 dB 15 minutes103 dB 7.75 minutes106 dB 3.75 minutes
How big is 1dB sound file http://www.phys.unsw.edu.au/jw/dBNoFlash.htmlDate Accessed 27/03/2008
Nightclub at 106dB
4 minutes unprotected2 hours protected
Gunshot 140dB
never unprotected1.5 minutes protected
Is your music too loud?“Preliminary data on iPods and similar devices havefound lower maximum levels - above 100 decibels (thenoise volume of a chainsaw; risk of hearing damageafter two hours), but not higher than 115 decibels (afootball game in a loud stadium; risk of hearing damageafter 15 minutes)…To fully understand the potentialimpact of these devices, it is important to knowthat the
sound is travelling a tiny distance from your earbud toyour eardrum rather than being diffused in a footballstadium or concert arena.”
http://www.hearingconservation.org/docs/RealityCheckonMusicNIHL.pdfExtract from article by Gregory Mott, Washington Post. Date Accessed 26/03/2008
Ultrasound
What is the normal range of humanhearing?
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 boundariesbetween 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 theultrasound procedure. Why is this?
Good contact is important. Otherwise the
ultrasound waves will simply bounce offthe outer skin and no image will beobtained. It makes movement of thetransducer easier. It ensures a clearpicture 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 thansurgical 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 orwavelength – remember thespeed is always the same.
The frequency of radio waves is much
higher than the frequency of soundwaves.
Imagine a wave where 2 waves pass apoint in 1 second.
frequency f = 2 Hz.amplitude
distance (m)
0.1 0.2
10
seconds in taken timeproduced waves of number
frequency=
10
What is the length of the wave producedin 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…..
speedamplitude
distance (m)
0.1 0.2
timedistance
speed=
Calculating Wave Speed
There are two formulae which can be usedto calculate wave speed.
Speed is a distance in a given time.
Calculating Wave Speed
speed = frequency x wavelength
v = fλ
speed in m/s
frequency in Hz
wavelength in metres
How fast is a radio or TV wave?
Radio and TV signals are part of the
electromagnetic spectrum.
This means they travel at the speed of
light which is 3 x 108 m/s.
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, angleof reflection and normal when a ray oflight is reflected from a plane mirror.State the principle of reversibility of a
ray path.
Reflection of LightWatch 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
reflectionComplete the sentences:
The normal is
The angle of incidence is
The angle of reflection is
Reflection of Light
Law of Reflection
angle of incidence = angle of reflection
Flash animation - reflection
ΘRΘi
The normal is at a right angle (90º) to the surface.
Reversibility of Rays
Law of Reflection
angle of incidence = angle of reflection
The normal is at a right angle (90º) to the surface.
Flash animation - reflection
RI
ΘR Θi
N
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 ofdirection 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 GlassBy the end of this practical session you
will know:
how to draw a normalhow to identify, and measure, angles of
incidence and refraction as lightpasses 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
Use a protractor to measure the angles of incidence (i) and refraction (r)
i =
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 happens when the ray of light enters the block
along the normal?
There is no change in direction of thelight 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 anglebetween 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 anglebetween the refracted ray and thenormal.
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.
Refraction: Air to Glass
normal
incident (or incoming) ray
refracted ray
angle of incidence i
angle of refraction r
r is less than i
RefractionLight 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 AirBy the end of this practical session you
will know:
how to draw a normalhow to identify, and measure, angles of
incidence and refraction as lightpasses 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 =
Angle ofIncidence (i)
Angle ofRefraction (r)
0o
(along the normal)
15o
35o
42o
75o
Refraction – Glass to Air
Ray Box
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?
What happens when the ray of light enters the blockalong the normal?
There is no change in direction of thelight i.e. it goes straight through.
What do you notice as you increase the angle ofincidence?
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 orabove?
Total internal reflection occurs and no
light escapes.
Refraction – Glass to Air
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 fibretransmission system.
Think!
What happens when light passes from one
material to another?
What happens when light passes fromglass to air? How does the angle ofincidence compare with the angle ofrefraction?
Critical Angle
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.
Flash animation
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
reflectionComplete the sentences to give definitions of
the normal, and angles of incidence andreflection.
The normal is
The angle of incidence is
The angle of reflection is
Reflection of Light
Law of Reflection
angle of incidence = angle of reflection
Flash animation - reflection
ΘRΘi
The normal is at a right angle (90º) to the surface.
Reversibility of Rays
Law of Reflection
angle of incidence = angle of reflection
The normal is at a right angle (90º) to the surface.
Flash animation - reflection
RI
ΘR Θi
N
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?
The sound signal must be changed into anelectrical signal. The signal is a series ofelectrical pulses.
What input device is used to convert sound to
an electrical signal?
The electrical signal is converted intopulses of light which are transmittedthrough the optical fibre via total internalreflection.
Transmission
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 toconvert an electrical signal to sound?
Receiving
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 frominterference.
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 normalelectric light bulb, what energytransformations 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!
End probe containing coherent bundle, incoherent bundle, lens and surgical instruments
Controls
Eyepiece
Light injected here
ENDOSCOPE
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
Light travels along both of these bundles by TOTAL INTERNAL REFLECTION
The purpose of the LIGHT GUIDE is to send light (but not heat!) INTO the
patientThe IMAGE GUIDE brings light back,
allowing the doctor to see inside the body
Key words: curved reflectors, receivedsignals, 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 curvedreflectors 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.
receiving dish aerial
rece
iver
If we tilt the dish…
receiver
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
transmitter
Dish Aerials
What is the advantage of using a dishwith a larger area?
The dish collects more of the incomingbeam and focuses it – resulting in astronger 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, 3.28McCormick & Baillie Int 2 Physics p133 qu 2, p145-146 qu 1, 2
YPQ2 3.25, 3.35McCormick & 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 anddiverging lenses.Describe the effect of converging anddiverging lenses on parallel rays of light.
Carry out calculations involving the
relationship between power and focal length of
a lens.
Lenses
Lenses refract light.
A convex lens is one which is thicker in
the middle than at its edge. It bulges out.
LensesWhen we shine parallel rays through a
convex lens…normal
Convex lenses bring parallel light rays to a focus. They are converging lenses.
Lenses
normal
The focal length of the lens is the
distance between the centre of the lens
and the focus.
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 isthinner 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!
Focal length f
Diverging lenses have a negative focal length.
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
Because the focal length is in the opposite direction, we give it a
NEGATIVE value
f
Summary (so far)
Type of lens
What it doesFocal length
Convex
CONVERGES light (brings the rays together)
Positive
Concave
DIVERGES light (spreads the rays)
Negative
Thick and Thin Lenses
Observe the demonstration with the lenses.
Can you predict which lenses will beconverging? And which will be diverging?
Is there a relationship between thethickness of the lens and the focallength?
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 thecentre 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 makingmeasurements?
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
dioptres.
lengthfocalpower
1
=
POWER OF A LENS
thfocal lengpower
1=
To calculate a lens’ power, use this equation:
fP
1=
For short:
METRES
(m)DIOPTRES
(D)
Power of Lenses
Converging lenses have a positive focal
length and a positive power.
Diverging lenses have a negative focal
length and a negative power.
Summary
Type of lens
What it doesFocal length
Power
Convex
CONVERGES light
(brings the rays
together)
Positiv
e
Positive
Concave
DIVERGES light
(spreads the rays)
Negativ
e
Negative
A lens has a focal length of 20 cm.Find its power.
What do I know?f = 20 cm = 0.2 m
Df
P 52.0
11+===
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 itsfocal length.
What do I know?f = ?P = -12 D
mP
f 083.012
11−=
−==
Focal Length and Power Questions
For each of the lenses below calculate the focal length or powerand state whether the lens is convex (converging) or concave(diverging).
1 Lens power +10D2 Lens power –10D3 Lens power +100D4 Lens power -24D5 Lens power 3D6 Lens power 16D7 Lens power 24D8 Lens power -32D9 Lens power -1.33D
10 Focal length 0.5 m11 Focal length -50cm 12 Focal length 20cm13 Focal length 0.2cm14 Focal length 0.4cm15 Focal length -0.25cm16 Focal length -0.10cm17 Focal length 30cm 18 Focal length -15cm
Which is the most powerful lens? Which has the longest focal length?
Find the focal length of 1-9Find the power of 10-18
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 converginglens forms the image of an object placed atdifferent 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 andshort 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 F
Virtual Int 2 Physics – Waves and Optics
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
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.
F
Source of light between one and two focal lengths
from the lens
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.
F
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 FF
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 FF
The image is upright and magnified.
F FF
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) ordiminished (smaller).
object of lengthimage of length
ionmagnificat =
Summary
Objects more than two focal lengths:The image is real, inverted and diminished.
Objects between one and two focallengths:The image is real, inverted and magnified.
Object less than one focal length:The image is virtual, upright and magnified.
The Human EyeLight 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
1
2
3
4
1
2
3
4
1 controls the amount of light which enters the eye
It is called
the iris
1
2
3
4 2 is the hole in the middle of the iris. It allows light to enter the eye.
It is called
the pupilIn bright light will the pupil be large or small?It will be smaller to prevent too much light from getting in to the eye.
1
2
3
4 Along with the cornea, 3 focuses the light. It is called
the jelly lens
How does the eye lens focus on objects at different distances?Muscles around the lens make it thicker or thinner.
1
2
3
4 4 is where the “light picture” is built up. It is called
the retina
The retina contains nerve cells which are affected by light and send signals to the brain along the optic nerve.
1
2
3
4 4 is where the “light picture” is built up. It is called
the retina
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 therefraction occurs in
the cornea –not the lens!
Common Sight Defects
Long and short sightare caused by the failure ofthe eye’s lens to bring thelight rays to a focus on theretina.
Long and Short Sight
Observe the demonstration of long and shortsight.
Where do the rays meet when a person is longsighted?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 SightA 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 SightRemember that rays do not actually pass beyondthe back of the eye!
Long sight can occur because the eyeball isshorter than normal from front to back. It canalso be caused by the ciliary muscles notrelaxing to make the lens fat enough.
Long SightLong sight can occur because the eyeball is
shorter than normal from front to back. It can
also be caused by the ciliary muscles notrelaxing 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 awayobjects clearly. The rays are parallel and can be
brought to a focus on the retina.
Objects close up appear blurred. The rayscoming 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 SightA 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 theeye 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 rayscoming 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.
