Transcript Waves and Radiation

Behaviour of Waves
Lesson 1: Types of Waves
Lesson 2: Wave Speed
Reflection
Lesson 3: Refraction
Total Internal Reflection
Lesson 4: Refraction in Water
Lesson 5: Refractive Index
Lesson 6: Refraction in Water
Lesson 7: Refractive Index
Lesson 8: Diffraction
Lesson 9: Transmitting Information
Lesson 10: Sound
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Syllabus Double Award
Properties of waves

describe longitudinal and transverse waves in ropes, springs and water where appropriate

state the meaning of amplitude, frequency, wavelength and period of a wave

recall that waves transfer energy and information without transferring matter

recall and use the relationship between the speed, frequency and wavelength of a wave:

wave speed = frequency × wavelength

v=f×λ

use the relationship between frequency and time period:

frequency = 1 / time period

f=1/T

use the above relationships in different contexts including sound waves and electromagnetic waves
Light and sound

recall that light waves are transverse waves which can be reflected and refracted

recall that the angle of incidence equals the angle of reflection

construct ray diagrams to illustrate the formation of a virtual image in a plane mirror

describe experiments to investigate the refraction of light, using rectangular blocks,

semicircular blocks and triangular prisms

recall and use the relationship between refractive index, angle of incidence and angle of refraction

n = sin (i) / sin (r)

describe an experiment to determine the refractive index of glass, using a glass block

describe the role of total internal reflection in transmitting information along optical fibres and in prisms

recall and use the relationship between critical angle and refractive index

sin c = 1 / n

recall that sound waves are longitudinal waves which can be reflected

recall that the frequency range for human hearing is 20 Hz – 20 000 Hz

describe how to measure the speed of sound in air by a simple direct method
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Syllabus Separate Science
Properties of waves

describe longitudinal and transverse waves in ropes, springs and water where appropriate

state the meaning of amplitude, frequency, wavelength and period of a wave

recall that waves transfer energy and information without transferring matter

recall and use the relationship between the speed, frequency and wavelength of a wave:

wave speed = frequency × wavelength

v=f×λ

use the relationship between frequency and time period:

frequency = 1 / time period

f=1/T

use the above relationships in different contexts including sound waves and electromagnetic waves

understand that waves can be diffracted through gaps or when they pass an edge, and that the extent of
diffraction depends on the wavelength and the physical dimension of the gap
Light and sound

recall that light waves are transverse waves which can be reflected, refracted and diffracted

recall that the angle of incidence equals the angle of reflection

construct ray diagrams to illustrate the formation of a virtual image in a plane mirror

describe experiments to investigate the refraction of light, using rectangular blocks, semicircular blocks and triangular prisms

recall and use the relationship between refractive index, angle of incidence and angle of refraction

n = sin(i) / sin(r)

describe an experiment to determine the refractive index of glass, using a glass block

describe the role of total internal reflection in transmitting information along optical fibres and in prisms

recall the meaning of critical angle c

recall and use the relationship between critical angle and refractive index

sin c = 1 / n

understand the difference between analogue and digital signals

recall that sound waves are longitudinal waves which can be reflected, refracted and diffracted

recall that the frequency range for human hearing is 20 Hz – 20 000 Hz

describe how to measure the speed of sound in air by a simple direct method

understand how an oscilloscope and microphone can be used to display a sound wave

use an oscilloscope to determine the frequency of a sound wave and appreciate that the pitch of a sound depends
on the frequency of vibration

appreciate that the pitch of a sound depends on the frequency of vibration of the source

appreciate that the loudness of a sound depends on the amplitude of vibration
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Types of Waves
08/04/2016
Aims
 To define the properties of waves
 To identify the types of waves
Starter: True or False
1)
2)
3)
4)
5)
6)
Sound travels faster than light
Sound travels faster in solid objects than in liquids or air
Echoes are caused by sound waves bouncing of walls
Sound waves are caused by the vibration of molecules
Sound travels faster at the top of Mt. Everest
Light waves bounce off walls
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A Wave


Waves transfer energy from one place to
another
The disturbance moves along the wave not the
particles
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A wave in the sea:
• disturbance is molecules of water going up and down
• medium = water
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A Mexican Wave


The Disturbance is the people going up and down.
The medium is the crowd.
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Count the number of complete
vibrations
in ten
Frequency
= 1 Hertz
seconds and then work out the frequency of this wave
Frequency = 1.4 Hertz
Ten second timer
Frequency = 0.5 Hertz
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Note that the waves are carrying energy from one
place to another but the water particles are not
transferred.
previous
next
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Frequency




Number of waves per second
Either passing one point or generated
Symbol: f
Units: Hz (s-1)
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Period




This is the duration of a wave
How long it takes for one wave to occur
Symbol: T
Units: s
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We often need to analyse waveforms produced b
scientific instruments.
For example:- A Cathode Ray Oscilloscope (CRO
is used to monitor heartbeats.
Steady Steve
1 second
0.5seconds Helen Heart-attack
David Deadman
Heart monitor
time
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Pictures of waves (waveforms)
1 mains voltage waveform
2 tuning fork
0.02 seconds
2.5milliseconds (0.0025second
3 Ultrasonic scanner 10 micro seconds
(0.00001 seconds)
Work out the frequency for these 3 waveforms
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Which animals made these sounds?
Pictures of waves (waveforms)
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1 Cow
0.004 seconds
2 Baby crying
0.5milliseconds (0.0005seconds)
3 Bat
10 micro seconds
(0.00001 seconds)
Pictures of waves (waveforms)
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Wavelength
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Wavelength




The length of one complete wave
peak to peak or trough to trough
Symbol: λ
Units: m
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Which of the points is exactly one wavelength
away from point A?Answer is E
A
B
E
C
F
G
D
The next 5 tasks will test to see if you have
understood the idea of wavelength
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1 Match the highlighted letter to one which is e
one wavelength away from it.
A B C D E F G H I J K L M N O P Q R S T U V WX
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2 Match the highlighted letter to one which is e
one wavelength away from it.
A B C D E F G H I J K L M N O P Q R S T U V WX
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3 Match the highlighted letter to one which is e
one wavelength away from it.
A B C D E F G H I J K L M N O P Q R S T U V WX
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4 Match the highlighted letter to one which is e
one wavelength away from it.
A B C D E F G H I J K L M N O P Q R S T U V WX
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5 Match the highlighted letter to one which is e
one wavelength away from it.
A B C D E F G H I J K L M N O P Q R S T U V WX
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Amplitude




maximum disturbance caused by a wave
measured from the middle to a peak or trough
symbol: a
units: m
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Questions about amplitude
Match the waveforms to the sounds
A
B
C
F
D
1 an echo
2 a crescendo
3 a weak regular heartbeat
4 a strong regular heartbeat
5 a symbol being struck
Click for
answers
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Transverse Waves
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Transverse Wave
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Transverse Wave


The disturbance is perpendicular to the direction of
movement
Example Light
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Longitudinal Waves
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Longitudinal Waves




The disturbance is parallel to the direction of movement
Aka: pressure, compression waves
Example sound
Sketch on board
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Wave Behaviour
recap
Aim
 To define the properties of waves
 To identify the types of waves
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Wave Speed
08/04/2016
Aim:
 To calculate the speed of waves
 To Revise Reflection
Starter:
•
•
Draw a transverse wave in rough and label the
wavelength and amplitude
The frequency of a wave is 5 Hz what is it’s Period?
(check back in your book if you need to)
• How fast the wave is travelling
• Symbol v
• Units m/s
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ENERGY
speed
VIBRATIONS
The speed is how fast the disturbance travels
(how fast the energy is transferred)
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speed
ENERGY
speed
Notice that both waves have the same speed bu
different wavelengths and frequencies. The wav
equation can explain this mathematically.
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Calculating the Speed of Wave
Speed = Distance
Time
An explosion happens 1000m away and takes 3
seconds to reach you what is the speed of sound?
Speed = Distance
Time
Speed = 1000
3
= 333 m/s
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Wave speed, wavelength and frequency are
related by the wave equation
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The Wave Equation
The wave equation relates the speed of the wave to its
frequency and wavelength:
Wave speed (v) = frequency (f) x wavelength ()
in m/s
in Hz
in m
V
Worksheet
f

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Practice Questions:
Write out the variables (ie frequency = …) and the equations used
1.
A water wave travels through a pond with a frequency of 5Hz and they are 10
m apart. How fast are they travelling?
2.
Ricky hears a thunderclap that makes his ear drum vibrate at 165 Hz. The
wavelength of the sound is 2 m how fast is the sound wave travelling?
3.
Hannah sings an A, she knows the wavelength of the sound wave is 1.5m. How
many times does her vocal chords vibrate per second? (hint: you need the
answer to question 2)
4.
Hannah plays another note on her guitar, the string vibrates with a frequency
of 300 Hz what is the duration (the period) of each vibration of the string?
5.
Jon has been running his heart is beating 180 times a minute. What is the
frequency and period of his heart beats.
6.
Jack throws a stone into the centre of a pond. The pond has a radius of 4m.
He notices that it takes 2 seconds for the wave to reach the edge. He thinks
the wavelength of the ripple is about 1m. What is the frequency of his
ripple? What is the period of the ripple?
7.
Purple light has a wavelength of around 6x10-7m. If its frequency is 5x1014
Hz what is the speed of light?
8.
Red light travels at the same speed. Work out its frequency if its wavelength
is about 4x10-7m.
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Reflection
Angle of incidence = Angle of reflection
Normal
Reflected ray
Incident ray
Angle of
incidence
Angle of
reflection
Mirror
Movie Worksheet
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Ultrasound
generator
high frequency
electrical signal
transducer
changes the
electrical
signal into
an ultrasonic
pulse
Contact
gel
monitor
Echoes are changed back into
electrical
signals. These are
processed and imaged on the
monitor
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Ultrasound
generator
(4.5 months)
monitor
•speed of ultrasound inside the body
•time for the ultrasound to travel
•and partly reflected ultrasonic waves from
boundaries can be processed to produce an
image of the foetus
Click to start the scan
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Wave Speed
recap
Aim:
 To calculate the speed of waves
 To Revise Reflection
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Refraction
08/04/2016
Aims:
 To demonstrate refraction
 To explain the cause of refraction
 To observe total internal reflection
Starter:
Why does the magic trick of the Disappearing Coin work?
Home
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Refraction at a Boundary
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Refraction
Refraction is when waves ____ __ or slow down due to
changing _________. A medium is something that waves
will travel through.
Movie
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Method:
Refraction
through
a
glass
block:
1) Place a rectangular block of glass or Perspex in the middle
of a page on your book and draw around it.
2) Draw a normal line a third along one of the long sides
3) Draw a line at 20° to this normal to mark the incident ray.
4) Use a ray box and single slit to shine a ray of light along
the path you have marked for the incident ray.
5) Mark two dots on the path of the refracted ray with a
pencil. Join the dots with a ruler to show the path of the
refracted ray.
6) Remove the block and draw the
path taken by the ray of light
inside the block.
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This wheel continues to
move fast
This wheel
slows first
The cars
direction
changes
This wheel
speeds up
The car
travels
slower on
the sand
than tarmac
The cars direction
changes
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Refraction through a glass block:
Wave slows down and bends
towards the normal due to
entering a more dense medium
Wave slows down but is
not bent, due to entering
along the normal
Wave speeds up and bends
away from the normal due to
entering a less dense medium
Movie Worksheet
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What is total internal reflection?
Method:
1) Place a semi circular block of glass or
Perspex in the middle of a sheet of
plain paper and draw around it.
2) Draw a normal line at the middle of the
flat side
3) Use a ray box and single slit to shine a
ray of light to the normal line
4) Change the angle of the ray of light
until total internal reflection if found.
5) Then change the angle until the light is
refracted along the horizontal side.
6) Mark two dots on the path of the
refracted ray with a pencil. Join the
dots with a ruler to show the path of
the refracted ray.
6) Measure the critical angle
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Total internal reflection – simulation
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Why is angle of incidence important?
If the angle of
incidence is
smaller than the
critical angle,
then the light
ray is refracted.
If the angle of
incidence equals the
critical angle, then
the light ray is
refracted along the
boundary.
If the angle of
incidence is greater
than the critical angle,
then total internal
reflection occurs.
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Is the critical angle always the same?
The critical angle is the smallest angle of incidence at which total
internal reflection occurs.
Different materials have a specific value for the critical angle:
Material
Critical angle
water
49°
acrylic plastic
42°
glass
41°
diamond
24°
Diamond has the lowest critical angle at 24°.
This means that diamond reflects more light than the other
materials and accounts for its characteristic sparkle.
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Total Internal Reflection
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How do optical fibres carry light?
Optical fibres do not have to be straight to carry light and can even
carry light around corners.
This curved Perspex
rod shows how light
travels in an optical
fibre.
Light travels through the Perspex rod, and optical fibres, by a
process called total internal reflection.
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Total Internal Reflection recap
Aims:
 To describe the effect of the critical angle
 To know some uses of the critical angle
Home
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Refraction in Water
08/04/2016
Aim
 Experimentally determine the effect of water depth
on speed and establish if there is a relationship
between the two
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Experiment: Wave Speed
You are expected to:
 Write a method
 Draw a graph of results
 Write a analysis / conclusion
 Write an evaluation
Overview:
 Work out what you are going to do before you start
 You need at least 6 different depths for a graph
 The effect is only noticed on small depths (mm not cm)
 You need to make sure that the effect is not drowned out by
your reaction time
 You need to make sure that one mistake does not ruin your
results
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Refractive Index
08/04/2016
Aim
 Experimentally determine the refractive index of glass
 describe an experiment to determine the refractive
index of glass
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Finding the Refractive Index of Glass
Method:
 Draw around the semi circular block.
 Draw a normal line in the middle of the straight edge of the block.
 Draw Incident lines every 10° from 0° to 70°
 Use a ray box to send incident rays into the semi circular box and
mark out the refracted rays.
 Measure the refracted angles
Results:
 Create a table of the incident, refracted angles, sin i, and sin r.
 Plot a graph of sin i (x) against sin r (y)
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Work To Do:
Results:
 Create a table of the incident, refracted angles, sin i,
and sin r.
 Plot a graph of sin i (x) against sin r (y)
Analysis
 What shape is your graph?
 Add an extra column to your table with the heading
refractive index (n)
 n = sin(i) ÷ sin (r)
 Calculate the gradient of your graph
 The critical angle of glass (c) = 41°
 Calculate sin(c)
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Equations to Learn:

A measure of how much light is refracted (bends) in a
material.
 glass n = 1.6
 Diamond n = 2.5
Refractive index (n):
n = sin i
sin r
Critical Angle (c):
sin c = 1
n
incident angle
refracted angle
Critical angle
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Refractive Index
recap
Aim
 Experimentally determine the refractive index of glass
 describe an experiment to determine the refractive
index of glass
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Diffraction
Separate Science
08/04/2016
Aims:
 To define diffraction
 To see reflection and refraction in water waves
Starter:
Worksheet
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Separate Science
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Separate Science
Water waves
are reflected
by the barrier
Barrier
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Separate Science
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Ripple tank
model
Separate
Science
The waves slow
down
when they reach the
shallow water
Deep water
The wavelength gets
shorter
But the frequency
stays the same
Shallow water
Can you understand
what is happening
here? HINT remember
the wave equation
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Water waves
Deep water
Separate
Science
If the waves
reach
the
shallow water at an
angle the waves still
slow down.
The waves also
change direction.
We call this effect
refraction.
Shallow water
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Separate Science
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Separate Science
• Diffraction is the spreading out of waves at
edges and gaps.
• All types of waves can be diffracted.
• The amount of diffraction is affected by the
size of the gap and the wavelength.
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Separate Science
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Separate Science
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Separate Science
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Separate Science
Small amount of
spreading (diffraction)
The wavelength of the
waves is much smaller
than the size of the gap
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Separate Science
The wavelength is the
same size as the gap
so there is very good
spreading (diffraction)
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Separate Science
For Edges
Long wavelengths diffract more than short
wavelengths.
For Gaps
The wavelength should be the same size
as the gap to get good diffraction.
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Separate Science
Speed of sound in air = 330m/s
Typical frequency = 440Hertz
Work out the wavelength of these sound waves
Speed = wavelength x frequency
Wavelength = speed/frequency
Wavelength = 330/440
= 0.75metres
About the same size as the width of a door
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Separate Science
To get spreading (diffraction) of the light
the gap it passes through has to be very
small-because the wavelength of light is
very small
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Separate Science
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Separate Science
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Separate Science
The aerial is on a pole
to try and improve the
reception of radio waves.
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Separate Science
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Separate Science
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Separate Science
There is very little diffraction of short wavelength
radio waves when they pass between two hills
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Separate Science
Long wavelength radio waves are more likely to be
diffracted when they pass between hills and are
more likely to be detected by the aerial.
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Separate Science
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Separate Science
Copy and complete this email:
To [email protected]
You are having difficulty with the signal strength on your
short wave radio because…
Helpful words/phrases to use: long wavelength
short wavelength, diffracted, gap, edge.
Copy and complete the diagrams to illustrate your
answer.
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B
Separate Science
A
C
D
Which of these show how rays are
(a) reflected?
(b) diffracted at a gap?
(c) refracted?
(d) diffracted at an edge?
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Separate Science
Which of these show how waves are
(a) reflected?
(b) diffracted at a gap?
(c) refracted?
(d) diffracted at an edge?
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Diffraction
recap
Aims:
 To define diffraction
Worksheet
Home
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Separate Science
Transmitting Information 08/04/2016
Aim
 To explain the difference between analogue and digital
signals
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Separate Science
What is an analogue signal?
Most of the signals sensed by humans are analogue signals. Everyday
examples include sound, light and temperature.
voltage
An analogue signal may have any value within a
continuous range.
time
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What is a digital signal?
Separate Science
Modern communication systems carry information from
analogue signals as digital signals.
current
A digital signal only contains two values: ‘0’ (off) and ‘1’ (on).
These are used to encode analogue information.
time
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Separate Science
Converting analogue to digital
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Separate Science
Why do analogue signals lose quality?
All types of signals lose strength as they travel. This is called
attenuation and means that signals often have to be amplified so
they can be used.
Signals can also pick up noise (interference).
original
attenuated
amplified
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Separate Science
Why don’t digital signals lose quality?
Digital signals also lose strength and need to be amplified.
original
attenuated
amplified
threshold
regenerated
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Digital vs Analogue



Analogue signals are continually changing.
Digital signals are either zero or one
This means if the signal gets distorted you can work out
the digital signal (it should either be zero or one) but
you will not be able to tell the original analogue signal
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Analogue or digital?
Separate Science
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Separate Science
Communications – matching activity
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Opinions on analogue and digital
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Separate Science
What’s in a cable of optical fibres?
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Separate Science
How do messages travel along optical fibres?
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Sending messages
Separate Science
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Separate Science
Optical fibres – true or false?
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Transmitting Information
recap
Aim
 To explain the difference between analogue and digital
signals
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Sound
08/04/2016
Aim
 Measure the Speed of Sound
 Know the human hearing range
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•Which animals hear the lowest and the highest
frequencies?
•Which animal has the largest hearing range?
100,000
10,000
1,000
frequency
100
(Hz)
10
1
0
human
dog
bat elephant mouse
dolphin
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Sound




The human hearing range is from 20 – 20 000 Hz
Its best range is at age 8
the pitch of a sound depends on the frequency of vibration of the
source
the loudness of a sound depends on the amplitude of vibration
A
B
Which sound is:
•The quietest
•The loudest
•The highest
•The lowest
C
D
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Speed of sound experiment
This investigation to calculate the speed of sound
should be carried out in a quiet open space.
START
STOP
00:0034
00
100 m
1. When you see the cymbals crash, press START.
2. When you hear the cymbals crash, press STOP.
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Speed of sound experiment – results table
Look at the results of the sound experiment.
Experiment
distance
(m)
time
(s)
speed
(m/s)
1
100
0.34
294
How are these values used to estimate the speed of sound?
distance
100
=
speed =
time
0.34
= 294 m/s
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Speed of Sound Experiment





Write a method for our speed of sound experiment
Write out the results
Calculate the speed of sound
Are our results accurate?
Why might they not be?
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Movie CB
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