Demonstrate understanding of aspects of wave behaviour

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Demonstrate understanding of aspects of wave
behaviour
Physics A.S. 1.4
2013
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Waves

• Waves are a means of transferring energy through
  a medium without transferring matter
• Light and Sound travel as waves, in straight
  lines
• Speed of light 300,000Km/sec
• Speed of sound 300m/sec
• Electromagnetic waves form a large spectrum from
  radio to gamma rays

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Electromagnetic Spectrum

• The EMR energy increases as the wavelength gets
  shorter
• Low energy communication, High energy hazard
  to life

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Waves

• There are many types of wave, but they are all
  either
• Longitudinal particles vibrate/oscillate
  parallel to the direction of the wave (eg Sound)
• Transverse particles vibrate/oscillate at 90o
  to the direction of the wave (eg water)
• The involvement of particles means these are
  mechanical waves
• EMR waves e.g light are transverse waves that do
  not need a medium (no particles are involved)

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Understanding waves
Transverse waves
Longitudinal waves compression and rarefaction of
particles
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What is Light?

• Light is a form of ____.
• It is a specific ______ on the electromagnetic
  spectrum.

• Words Wavelength, energy, colour

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Properties of Light

• Travels in straight lines
• _____km/h (in a vacuum)
• How many m/s (in S.F.)?
• If light passes through a medium (material) it is
  called _________
• If it is scattered it is called ________
• If it is blocked it is called _________

3x108 ms-1

• Transparent, Opaque, 300000, Translucent,
  300000000

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Some definitions
1) Amplitude measured from mid position to
crest or trough
2) Wavelength (?) this is the distance between
two corresponding points on the wave and is
measured in metres
3) Frequency this is how many waves pass a
point every second and is measured in Hertz (Hz)
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Terminology

• Peak
• Trough
• Amplitude
• Wavelength (?)
• Period (T) time taken for the wave to travel 1
  cycle OR time for a wave to pass a point,
  measured in seconds

1 cycle
TIME
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Sound

• Sound is not EMR it involves particles.
• All EMR waves are transverse waves.
• BUT sound is a longitudinal wave that compresses
  the medium through which it is transmitted.
• More particles compressed (higher amplitude)
  means more volume

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Sound
We hear things when they vibrate. If something
vibrates with a high frequency (vibrates very
______) we say it has a _____ pitch. If something
vibrates with a low frequency (vibrates ______)
we say it has a ____ pitch. Words slowly, low,
high, quickly Higher frequency Higher pitch
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Frequency
This sound wave has a _____ frequency
high
This sound wave has a ___ _frequency
low
What is the relationship between frequency and
wavelength?
Lower frequency higher wavelength
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Amplitude
This sound wave has a _____ amplitude (loud)
high
Amplitude energy (volume, brightness etc)
This sound wave has a _____ amplitude (quiet)
low
Sound video
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Sound is a longitudinal or compression wave.

• Anything vibrating causes longitudinal waves.
• The particles vibrate back-and-forth creating
  sound.
• Sometimes shown as up and down, or sine waves -
  easier to understand.

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http//www.kettering.edu/drussell/Demos/waves-int
ro/waves-intro.html
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Draw these waves
1) Quiet sound, low frequency (i.e. high
wavelength)
2) Quiet sound, high frequency (i.e. low
wavelength)
3) Loud sound, low frequency
4) Loud sound, high frequency
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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
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Questions

• Sound travels at 330m/s. If a sound note is
  played at 484Hz find its wavelength.
• 0.68m (2-3 sig fig)
• The speed of light is 3.0x108 m/s and the
  wavelength of red light is 650x10-9m. Find the
  frequency of red light.
• 4.6x1014 Hz
• 4 complete water waves pass a point every 2 secs.
  The distance between 6 crests is 12m. Find the
  speed of the wave.
• 4.8m/s (draw the waves out)

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Some example wave equation questions

1. A water wave has a frequency of 2Hz and a
   wavelength of 0.3m. How fast is it moving?
2. A water wave travels through a pond with a speed
   of 1m/s and a frequency of 5Hz. What is the
   wavelength of the waves?
3. The speed of sound is 330m/s (in air). When Dave
   hears this sound his ear vibrates 660 times a
   second. What was the wavelength of the sound?
4. Purple light has a wavelength of around 6x10-7m
   and a frequency of 5x1014Hz. What is the speed
   of purple light?

0.6m/s
0.2m
0.5m
3x108m/s
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Period and Frequency

• Period (T) time for a wave to pass a point,
  measured in seconds
• Frequency how many waves pass a point every
  second
• Period and frequency are related by the equation
• F1/T
• F is frequency (Hz)
• T is period (s)

frequency (f) 1 / period (T)
in Hz in s
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Questions

• What is the period of a 330Hz wave?
• 0.003s
• What is the frequency of a wave with period of
  0.4s?
• 2.5Hz
• Complete the questions pg 113, 114 ESA study
  guide
• Video properties of waves

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Reflection

• All waves can reflect.
• In a plane (flat) mirror images are reflected
  about the normal. (an imaginary line 90o to the
  mirror)
• Plane mirrors also laterally invert images.
• Plane mirrors produce virtual images (shown by
  dashed line).

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Reflection
Angle of incidence Angle of reflection
Normal
Reflected ray
Incident ray
Angle of reflection
Angle of incidence
Mirror
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Lateral Inversion

• Images in plane mirrors appear the same size and
  as far behind as in front but they are switched
  from side to side

Light from each hand reflects from the mirror
into our eyes. The eyes register the image in
direct line of sight. The persons left hand
appears to be the right. Why is the wave behind
the mirror dashed? Light is blocked by the
mirror so the image is virtual.
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How to draw reflection diagrams

• Draw the position of the reflected candle. (same
  size, as far behind as in front)
• Draw 2 rays from the image to the eye (line of
  sight)
• Image must be virtual (dashed lines)
• Use the law of reflection to show how light
  travels from the candle to the mirror. (normal
  line)
• Show the direction the rays move. (arrows in the
  centre)

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Refraction through a glass block
How does the brightness of a refracted ray differ
from a reflected one? Why?
Set up this experiment and complete the diagrams
Wave slows down but is not bent, due to entering
along the normal
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Refraction

• Rules
• Describe the optical densities of the media
• Frequency remains the same.
• State the change of speed (faster or slower)
• Therefore the wavelength changes (increase or
  decrease)
• So the refracted ray bends (towards or away from
  the normal)

LESS
MORE
?
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Refraction

• When going from less optical density to more it
  bends towards the normal.
• When going from more optical density to less it
  bends away from the normal.
• Refraction always involves a change in speed,
  direction and wavelength.

Optical density changes Air lt water lt glass lt
diamond
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Refraction
Refraction is the bending of waves. It happens
when waves ____ __, wavelength and direction due
to entering a _________ (substance) of different
optical density. When a pen is placed in water it
looks like this
In this case the light rays are slowed down by
the water and are _____, causing the pen to look
odd. The two mediums in this example are ______
and _______.
Draw a ray diagram to show why the pen appears
bent.
Words change speed, water, air, bent, medium
http//www.bcscience.com/bc8/pgs/quiz_section4.2.h
tm
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How to draw refraction diagrams

• Draw the image
• Draw 2 rays from the image to the viewer (line of
  sight)
• Draw 2 rays from the object to the points of
  refracted rays at the interface.
• Show the direction the rays move. (arrows in the
  centre)
• Draw a normal to check refraction is correct
• wiki

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Question

• White light travelling through air enters a more
  optically dense medium (plastic) and is refracted
  towards the normal because it changes its speed,
  direction and wavelength, as it exits the prism
  it enters a less optically dense medium (air) and
  again is refracted this time away from the
  normal. The shorter wavelength blue light travels
  faster and is bent more than the longer
  wavelengths so a spectrum is formed.

• Explain how the prism creates a spectrum.

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Finding the Critical Angle
1) Ray gets refracted
2) Ray still gets refracted
4) Ray gets internally reflected
3) Ray still gets refracted (just!)
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Total Internal Reflection

• If light goes from a more optically dense medium
  to a less optically dense one it is refracted
  away from the normal. Ti lt Tr
• When the angle of refraction reaches 90o the
  angle of incidence is known as the critical
  angle.
• Any further increase in the angle of incidence
  will result in the light being reflected rather
  than refracted.

• Rules
• More optically dense to less
• Ti Tc when refraction is at 90o
• Ti gt Tc therefore reflection

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Mirages

• The mirage is a reflection of the sky. Explain
  using the ideas of total internal reflection how
  the mirage occurs. In your answer consider how
  the optical density of the air changes as it gets
  hotter.

• Air near the road surface is heated and less
  optically dense. Light from the sky therefore
  travels from a more to a less optically dense
  medium and bends away from the normal. The Ti
  Tc when refraction is at 90o. In this case the Ti
  gt Tc therefore reflection of the sky occurs and
  the brain misinterprets it as water.

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Uses of Total Internal Reflection
Optical fibres An optical fibre is a long,
thin, _______ rod made of glass or plastic and
sheathed in a low optical density material.
When light hits the boundary the angle of
incidence is greater than the critical angle and
so is _______ reflected from one end to the
other, making it possible to send ____ chunks of
information
Optical fibres can be used for _________ by
sending electrical signals through the cable.
The main advantage of this is a reduced ______
loss.
Words communications, internally, large,
transparent, signal
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Other uses of total internal reflection
1) Endoscopes (a medical device used to see
inside the body)
2) Binoculars and periscopes (using reflecting
prisms)
Complete All questions in activity 8B pg 130 ESA
study guide
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Rainbows

• In order to see a rainbow the observer must be
  looking towards the rain with the sun behind
  them.

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Refraction and Dispersion

• White light can be refracted and dispersed into
  the visible spectrum.
• When light enters a more optically dense medium
  it refract, slows down, decreases wavelength and
  bends towards the normal.
• Red light slows less than violet and decreases
  its wavelength by less therefore it bends less
  towards the normal.
• When travelling into a less optically dense media
  the opposite occurs.

• In a prism light bends twice in the same
  direction (down and Down)

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Wave diagrams
1) Reflection
2) Refraction
4) Diffraction
3) Refraction
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Diffraction
Diffraction is the ability of waves to spread
from a gap or get around a barrier. (dont use
the term bending)
More diffraction if the size of the gap is
similar to the wavelength
More diffraction if wavelength is increased (or
frequency decreased)
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Low frequency sounds diffract more
http//hyperphysics.phy-astr.gsu.edu/hbase/sound/i
mgsou/difr.gif
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Diffraction over or around the hill

• If a man and woman are at the source whose voice
  would the child hear?
• Man has low pitch voice
• Low frequency, high wavelength because Vf?
• So mans voice diffracts more and is heard

CHILD
http//www.pa.op.dlr.de/acoustics/essay1/beugung_e
n.html
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Radio wave diffraction

• FM radio is high frequency
• v fx?
• Therefore short wavelength which cant diffract
  much so the house doesnt get reception

• AM radio is low frequency
• Therefore long wavelength which diffracts more so
  the house gets reception

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Sound refraction

• Sound travels faster in warm air.
• Lower part of wave front gets ahead of upper
  part, so front turns upward.

http//sol.sci.uop.edu/jfalward/physics17/chapter
10/chapter10.html
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ripple tank demo
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Sound reflections (echos)

• When sound waves hit a surface they are
  reflected.
• They are travelling in the same medium so they
  will have the same frequency, speed and
  wavelength BUT the direction will change.
• The amplitude of reflected wave is less as some
  of the energy is lost.
• V d/t

V speed (m/s) D distance (m) T time (s)
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Sonar can also be used to tell the different
layers beneath the sea floor.
http//www.coml.org/edu/tech/count/acoustic-tech-o
bs/bottom-penetrating-sounders/2-dositsreflec.gif
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Application - Echolocation in mammals

• Some animals use ultrasound to find out about
  their surroundings, navigate and find food and
  mates.
• High pitch, low energy clicks reflect off close
  objects.
• The animal detects and determines how far away an
  object is based on the time difference.
• Advantages
• High frequency, cant be heard by predators
• Short wavelength, more likely to reflect off
  rather than diffract
• Doesnt require much energy to produce the clicks
• Energy dissipates quickly, less interference of
  signal
• Disadvantages
• Energy dissipates quickly, no good for far away
  objects

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How does ultrasound work?
Ultrasound is the region of sound above 20,000Hz
it cant be heard by humans. It can be used in
pre-natal scanning
How does it work?
Ultrasonic waves are partly _________ at the
boundary as they pass from one _______ to
another. The time taken for these reflections
can be used to measure the _______ of the
reflecting surface and this information is used
to build up a __________ of the object.
Words depth, reflected, picture, medium
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Other uses of ultrasound
1) Echo sounding
The ultrasound is reflected from the sea floor.
2) Breaking down kidney stones
Ultrasonic waves break kidney stones into much
smaller pieces
3) Cleaning (including teeth)
Ultrasound causes dirt to vibrate dirt off
without damaging the object
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The Structure of the Earth
How do we know this? These facts have all been
discovered by examining seismic waves
(earthquakes)
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Seismic waves
Earthquakes travel as waves through the Earth
we call them SEISMIC WAVES. There are two types

• P waves
• They are longitudinal so they cause the ground to
  move up and down
• They can pass through solids and liquids
• They go faster through more dense material

• S waves
• They are transverse so they cause the ground to
  move from right to left
• They ONLY pass through solids
• They are slower than P waves
• They go faster through more dense material

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Seismic waves
The paths of these waves are all curved because
density is gradually changing
These observations tell us 3 things about the
Earth 1) It has a thin crust, 2) it has a
semi-fluid mantle where density increases with
depth, 3) a core with a liquid outer part and a
solid inner part.
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The electromagnetic spectrum
Each type of radiation shown in the
electromagnetic spectrum has a different
wavelength and a different frequency
Gamma rays X-rays Ultra violet Visible light Infra red Microwaves Radio/TV
Each of these types travels at the same speed
through a _______ (300,000,000m/s), and different
wavelengths are absorbed by different surfaces
(e.g. infra red is absorbed very well by
___________ surfaces). This absorption may heat
the material up (like infra red and _______) or
cause an alternating current (like in a __
_______).
Words black, microwaves, long, short, TV
aerial, vacuum
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How Microwaves and Infra-red work
Microwaves are absorbed by water molecules up to
a depth of a few centimetres. The heat then
reaches the centre of the food by conduction.
Infra-red waves are absorbed by the surface of
the material and the energy is then passed to the
centre of the food by conduction.
The higher the frequency of the wave, the greater
its energy
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X-rays and gamma (?) rays
X-rays are absorbed by ____ parts of the body,
like ____. Unfortunately, over-exposure to
x-rays will damage cells.
Gamma rays can be used to treat _______. A gamma
ray source is placed outside the body and rotated
around the outside of the tumour. Doing this can
___ the cancerous cells without the need for
______ but it may damage other cells and cause
sickness.
Tracers can also be used these are small
amounts of ___________ material that can be put
into a body to see how well an organ or ______ is
working.
Words radioactive, gland, cancer, hard, bones,
kill, surgery