1. 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.
Friday 29th February

2. Waves & Optics

3. What do you know?
About waves?
About optics?
Friday 29th February

4. frequency, wavelength, speed, amplitude, period.
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.
What words are new?

5. Waves & Energy Transfer
Waves transfer energy.
The energy transferred by waves can be
considerable!

6. Indian Ocean 2004

7. Indonesia 2005

8. Waves transfer energy…

9. Waves transfer energy…

10. 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.

11. What is meant by the crest of the wave?
The crest is the top part
of the wave

12. …and the trough?
The trough is the bottom
part of the wave.

13. What is the amplitude of the wave?
The amplitude is the
distance from the axis to
the crest

14. or from axis to trough.

15. Definition of Wavelength?
The wavelength is the
distance after which the
wave pattern repeats itself
– the distance between two
identical points on the wave

16. Wavelength is given the symbol λ pronounced lambda.

17. Frequency The frequency of the wave is the number
of waves each second.
It is measured in hertz (Hz) which just
means “per second”.

18. 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.

19. Period The period of a wave is the time taken
for one complete wave to pass a point.
It is measured in seconds (s).

20. Frequency & Period
The link between the frequency and
period of a wave

21. Frequency & Period
Rearranging

22. Longitudinal Waves
In this type of wave, particles vibrate back and forwards along the direction the wave is travelling.

23. Longitudinal Waves A sound wave is a longitudinal wave.

24. Transverse Waves
In this type of wave, particles vibrate at right angles to the direction of motion of the wave.

25. Transverse Waves
Light, and all forms of electromagnetic radiation, are transverse waves.

26. Homework for next lesson
Learn:
transverse and longitudinal waves
properties of waves
Axis
Crest
Trough
Amplitude
Wavelength
Frequency
Period

27. electromagnetic spectrum: gamma rays, X-rays,
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 m/s and that
light is also transmitted at this speed.

28. About the electromagnetic spectrum?
What do you know?
About waves?
About light?
About the electromagnetic spectrum?
Friday 29th February

29. Visible light is the only type of light
Myth or Reality?
Visible light is the only type of light

30. Myth! Visible light is a tiny slice of the radiation
that makes up the electromagnetic
spectrum.

31. What is radiation?

32. All radiation is harmful.
Myth or Reality?
All radiation is harmful.

33. Light is a form of radiation. All
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.

34. 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.

35. Where does light come from?

36. 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.

37. What is the electromagnetic spectrum?

38. The electromagnetic spectrum consists of all the different wavelengths of electromagnetic radiation, including light, radio waves, and X-rays.
All travel at

39. 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.

40. 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.

41. 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.

42. Microwaves 0.1 – 10 cm
Basis of almost all space communications.

43. Infrared 0.1 cm to cm
Heat!

44. Infrared wavelengths are about same size as a single bacteria.
Infrared 0.1 cm to cm
Infrared wavelengths are about same size as a single bacteria.
Heat!

45. 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.

46. Visible Light The Sun emits most of its
radiation in the visible range, which our
eyes perceive as the colours of the
rainbow.

47. 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.

48. Ultraviolet

49. 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.

50. 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.

52. Rabbits Radio Mambo Microwaves In Infrared Very Visible Light Unusual
Low frequency
Long wavelength
Rabbits
Mambo In
Very
Unusual
eXpensive
Gardens
Radio
Microwaves
Infrared
Visible Light UV
X-rays
Gamma Rays
High frequency
Short wavelength
All e-m waves travel at the speed of light!

53. Transverse Waves
In this type of wave, particles vibrate at right angles to the direction of motion of the wave.

54. Transverse Waves
Light, and all forms of electromagnetic radiation, are transverse waves.

55. What have you learned today?

56. speed of sound in air, using the and speed.
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

57. 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.
between speed, wavelength and frequency
for waves.

58. What do we know about sound?

59. Sound
How does sound travel?
How fast does sound
travel?

60. 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?

61. Energy Transfer
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?
Signal generator Loudspeaker

62. 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.

63. How fast does sound travel?
Why is an electronic timer used when
measuring the speed of sound?

64. Measuring Average Speed
Measure the time taken for sound to travel from one microphone to the other.
Repeat your measurements to improve reliability.

65. microphone
Electronic timer
metre stick
metal plate
hammer

66. 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?

67. 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?

68. 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?

69. 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.

70. 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

71. Longitudinal Waves
In this type of wave, particles vibrate back and forwards along the direction the wave is travelling.

72. Longitudinal Waves A sound wave is a longitudinal wave.

73. Loudness of Sound We measure the loudness of sound in decibels – dB.
Spending too long listening to loud sounds
can permanently damage your hearing.

74. 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.

75. Max recommended length of exposure 85 dB 8 hours 88 dB 4 hours
97 dB minutes
100 dB minutes
103 dB minutes
106 dB minutes
How big is 1dB sound file
Date Accessed 27/03/2008

76. Nightclub at 106dB
4 minutes unprotected
2 hours protected
Gunshot 140dB
never unprotected
1.5 minutes protected

77. 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.”
Extract from article by Gregory Mott, Washington Post. Date Accessed 26/03/2008

78. Ultrasound What is the normal range of human hearing?
20 Hz – Hz (or 20 kHz).
What is ultrasound?
Ultrasound is sound above Hz.

79. Imaging with Ultrasound
Medical ultrasound used for imaging a
foetus is between 1 and 20 MHz
( and Hz!)
How is ultrasound used for imaging?

80. Imaging with Ultrasound
A handheld transducer is held on the
stomach. This changes electrical energy
to sound energy which is transmitted in
waves into the body.

81. 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.

82. 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.

83. 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.

84. 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.

85. 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.

86. SOund NAvigation and Ranging
Using Sonar
SOund NAvigation and Ranging
Used for underwater ranging –
first patented in 1913 –
prompted by Titanic disaster.

87. Using Sonar
How does Sonar work?
What is it used for?
Sonar Use

88. 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.

89. wavelength – remember the
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.

90. Imagine a wave where 2 waves pass a point in 1 second.
frequency f = 2 Hz.
amplitude
distance (m)
0.1
0.2 10

91. speed 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)

92. Calculating Wave Speed
There are two formulae which can be used
to calculate wave speed.
Speed is a distance in a given time.

93. Calculating Wave Speed
speed = frequency x wavelength
v = fλ
speed in m/s
wavelength in metres
frequency in Hz

94. 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.

95. of reflection and normal when a ray 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.

96. Reflection of Light reflection? How do they compare?
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

97. 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

98. Reflection of Light Law of Reflection
The normal is at a right angle (90º) to the surface.
Law of Reflection
angle of incidence = angle of reflection Θi ΘR
Flash animation - reflection

99. Reversibility of Rays Law of Reflection
The normal is at a right angle (90º) to the surface.
Law of Reflection
angle of incidence = angle of reflection N I R ΘR Θi
Flash animation - reflection

100. What have you learned?

101. of refraction and normal. direction as light passes from air to glass
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.

102. Refraction

103. Rays of Light In any material (air, glass, water) light will
travel in a straight line.
We represent light using ray diagrams.

104. Drawing Light Rays We represent light using ray diagrams.
What two things must you remember
when drawing light rays?

105. Refraction of Light speed can cause it to change direction.
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.

106. 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

107. Activity – Air to Glass incidence and refraction as light
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.

108. Place the block on the outline provided

109. Shine the light ray along the line

110. Accurately mark the path of the refracted ray
Then switch off the light source and remove the block

111. Remove the block and draw in the refracted ray

112. Carefully draw in the normal

113. Mark the angles of incidence (i) and refraction (r)
Use a protractor to measure the angles of incidence (i) and refraction (r)

114. Record your results in the space provided
Use a protractor to measure the angles of incidence (i) and refraction (r)

115. Think! incidence? 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?

116. Think! refraction? incidence is greater than 0o?
What is meant by the angle of
refraction?
What happens when the angle of
incidence is greater than 0o?

117. 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.

118. 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.

119. r is less than i refracted ray angle of refraction r
angle of incidence i
normal
incident (or incoming)
ray
r is less than i

120. 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.

121. 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

122. Activity – Glass to Air incidence and refraction as light
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.

123. Place the block on the outline provided

124. Shine the light ray along the line

125. Accurately draw in the path of the refracted ray
Then switch off the light source and remove the block

126. Accurately draw in the path of the refracted ray
Then switch off the light source and remove the block

127. Carefully draw in the normal

128. r i Measure the ANGLE OF INCIDENCE, i
Measure the ANGLE OF REFRACTION, r

129. Record your results in the space provided

130. 0o 15o 35o 42o 75o Refraction – Glass to Air Angle of Incidence (i)
Refraction (r) 0o
(along the normal)
15o
35o
42o
75o
Ray Box

131. Think! incidence? 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?

132. Think! refraction? incidence is greater than 0o? What do
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?

133. 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.

134. 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.

135. Blue light refracts more than red light

136. Why does refraction occur?
Virtual Physics Animations -> Refraction of Waves

137. 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.

138. All lenses refract light
All lenses refract light. Lenses are used in cameras, telescopes and binoculars.

139. Total Internal Reflection and the Amazing Disappearing Coin

140. transmission system. 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.
by the “critical angle”.
Describe the principle of an optical fibre
transmission system.

141. Think! What happens when light passes from one material to another?
glass to air? How does the angle of
incidence compare with the angle of
refraction?

142. Critical Angle This is called the 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.

143. What happens at the critical angle?

144. 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.

145. 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

146. Reflection of Light Law of Reflection
The normal is at a right angle (90º) to the surface.
Law of Reflection
angle of incidence = angle of reflection Θi ΘR
Flash animation - reflection

147. Reversibility of Rays Law of Reflection
The normal is at a right angle (90º) to the surface.
Law of Reflection
angle of incidence = angle of reflection N I R ΘR Θi
Flash animation - reflection

148. Total internal reflection is used in fibre optics which are used in:
medicine ;
cable television ;
internet ;
telephone access.

149. 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.

150. 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?

151. 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.

152. 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?

153. What are the advantages of using optical fibres in communications?

154. 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!)

155. There is less loss of signal – so not as
many amplifiers required.
Disadvantage – can be more difficult to
join fibres than copper wires.

156. 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!

157. 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!

158. 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

159. 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.

160. Endoscopes

161. Light travels along both of these bundles by TOTAL INTERNAL REFLECTION
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

162. reflectors used in telecommunication.
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.
certain transmitted signals.
Describe an application of curved
reflectors used in telecommunication.

163. 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?

164. 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

165. 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.

166. If we tilt the dish…
receiver
receiver
receiving dish aerial

167. The transmitting aerial allows a strong
transmitter
transmitting dish aerial
The transmitting aerial allows a strong
(concentrated) signal to be sent in a particular
direction from the transmitting dish aerial.

168. transmitter
transmitting dish aerial

169. 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!

170. Dish Aerials A dish aerial can be used either to
receive or to transmit a radio (or
microwave) signal.

171. Tasks & Homework for … YPQ2 3.23, 3.28
McCormick & Baillie Int 2 Physics
p133 qu 2, p qu 1, 2
YPQ2 3.25, 3.35
p133 qu 1

172. What have you learned?

173. diverging lenses on parallel rays of light.
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 power and focal length of
a lens.

174. Lenses Lenses refract light. A convex lens is one which is thicker in
the middle than at its edge. It bulges out.

175. Lenses When we shine parallel rays through a convex lens…
normal
Convex lenses bring parallel light rays to a focus. They are converging lenses.

176. 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.

177. 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)

178. The distance from the centre of the lens to the principal focus is called the FOCAL LENGTH (f)

179. Lenses A concave lens is one which is
thinner in the middle than at its edge. It
“caves” in.

180. Lenses When we shine parallel rays through a concave lens…
normal
Concave lenses cause parallel light rays to diverge.

181. 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!

182. Diverging lenses have a negative focal length.
Focal length f

183. When light rays enter a CONCAVE lens they are DIVERGED (spread)
This time the principal focus is BEHIND the lens

184. 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

185. Summary (so far) Type of lens What it does Focal length Convex
CONVERGES light (brings the rays together)
Positive
Concave
DIVERGES light (spreads the rays)
Negative

186. Thick and Thin Lenses Can you predict which lenses will be
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?

187. 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.

188. 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.

189. 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?

190. Compare these 2 convex lenses:

191. The thicker lens has a shorter focal length

192. The thicker lens “bends” the light more, so we say it has a greater POWER than the thin lensf f

193. Power of Lenses A more powerful lens causes more refraction.
The power of a lens is measured in dioptres.

194. To calculate a lens’ power, use this equation:
POWER OF A LENS
To calculate a lens’ power, use this equation:
For short:
METRES
(m)
DIOPTRES (D)

195. 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.

196. Summary Type of lens What it does Focal length Power Convex
CONVERGES light (brings the rays together)
Positive
Concave
DIVERGES light (spreads the rays)
Negative

197. A lens has a focal length of 20 cm. Find its power. What do I know?
f = 20 cm = 0.2 m
What type of lens is this? How do you know?

198. A lens has a power of -12D. What type of lens is this?
Diverging / concave – since the power is negative.

199. A concave lens has a power of -12D. Find its
focal length.
What do I know?
f = ?
P = -12 D

200. Focal Length and Power Questions
For each of the lenses below calculate the focal length or power
and state whether the lens is convex (converging) or concave
(diverging).
Find the focal length of 1-9
Find the power of 10-18
1 Lens power +10D
2 Lens power –10D
3 Lens power +100D
4 Lens power -24D
5 Lens power 3D
6 Lens power 16D
7 Lens power 24D
8 Lens power -32D
9 Lens power -1.33D
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?

201. lens forms the image of an object placed at
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.

202. 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.

203. Creating Images with Lenses
2. If the light ray enters the lens travelling horizontally, it is refracted through the focal point.

204. Source of light relatively distant from the lens
Virtual Int 2 Physics – Waves and Optics

205. Source of light relatively distant from the lens
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.

206. Source of light more than two focal lengths from the lens
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.

207. Source of light between one and two focal lengths from the lens
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.

208. Source of light between less than one focal length from the lens
The rays do not meet.

209. We trace the rays back to find where the rays would meet.

210. 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

211. The image is upright and magnified.

212. 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.

213. Ray Diagrams Virtual Int 2 Physics -> Waves & Optics ->
Lenses -> Ray Diagrams & More Ray
Diagrams

214. Magnification of a Lens
An image formed by a lens is either
magnified (bigger) or
diminished (smaller).

215. 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.

216. 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.

217. The Human Eye 4 1 2 3

218. the iris 4 1 2 3 1 controls the amount of light which enters the eye
It is called
the iris 1 2 3 4

219. the pupil 4 1 2 3 In bright light will the pupil be large or small?
2 is the hole in the middle of the iris. It allows light to enter the eye.
It is called
the pupil 1 2 3 4
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.

220. 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.

221. 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.

222. 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.

223. Virtual Int 2 Physics -> Waves and Optics -> Applications of Lenses -> The Eye

224. Remember!
Most of the
refraction occurs in
the cornea –
not the lens!

225. Long and short sight Common Sight Defects are caused by the failure of
the eye’s lens to bring the
light rays to a focus on the
retina.

226. Long and Short Sight Observe the demonstration of long and short
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

227. 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.

228. 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
relaxing to make the lens fat enough.

229. 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.

230. 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.

231. 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.

232. 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.

233. 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.

234. 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.

235. 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.