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8/20/2019 Year 11 World Communicates
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Physics: The World Communicates
Rebecca Duong
PHYSICS: THE WORLD COMMUNICATES 1
Task Outcome 1: The wave model can be used to explain how current technologies transfer information.
Describe the energy transformations required in one of the following:
- mobile telephone
- fax/modem
- radio and television
Mobile Telephone:
Sound energy: we speak into the microphone of a phone with our voices
Electrical energy: our voice is transformed into digitized electrical signals (binary)
Electromagnetic energy: signals are transmitted as radio waves to a base station where a system of antennae on
towers or tall building accepts them
Electrical energy: EM wave is transformed back and runs through the base station where the signal is amplified
again and the base station act as a transmitter
Electromagnetic energy: antenna transmits the wave through the air again
Electrical energy: the other phone’s antenna captures the wave and converts it
Mechanical energy: the energy is converted into sound by the speaker in the phone
Describe waves as a transfer of energy disturbance that may occur in one, two or three dimensions, depending on the nature of
the wave and the medium
Waves are travelling vibrations or disturbances that transport energy without transporting matter
A pulse is a single disturbance travelling from one point to another
They are caused by a vibration or disturbance and transfer the disturbance and energy to other bodies but the
medium does not move forward
One dimension waves: wave along a line e.g. motion of a longitudinal wave in a slinky where the medium confines
the wave to the slinky so the energy of wave motion has only one dimension to travel
Two dimension waves: wave on a plane e.g. a pebble thrown into a still pond produces a transverse wave travelling
outward and away with a two dimension circular wavefront
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Three dimension waves: waves through a 3D space e.g. the point source of sound (e.g. when we speak) and light
(e.g. light from a lamp) result in waves that immediately travel away from the source in 3 dimensions with spherical
wavefronts
Identify that mechanical waves require a medium for propagation while electromagnetic waves do not
Medium is the material through which mechanical waves are propagated
Mechanical waves require a medium or material for their propagation since the transfer of energy occurs through
the motion of the particles in the medium
Electromagnetic waves do not require a medium for propagation since they self propagate through perpendicular
electric and magnetic fields
Define and apply the following terms to the wave model: medium, displacement, amplitude, period, compression, rarefaction,
crest, trough, transverse waves, longitudinal waves, frequency, wavelength, velocity
Medium: the material through which mechanical waves are propagated
Displacement: distance of a particle from its rest or equilibrium position (y-axis)
Amplitude: maximum displacement of a particle from its rest position and corresponds to a crest
Period: time taken for one complete wave to pass any point or one complete oscillation of a point on the wave
Compression: zones of higher pressure where the particles of the medium are pushed closer together
Rarefaction: zones of lower pressure where the particles of the medium are spread further apart
Crest: highest points on the wave
Trough: lowest points on the wave
Transverse waves: particles in the medium vibrate perpendicular to direction of propagation or energy transfer
Longitudinal waves: particles in the medium vibrate back and forth parallel to the direction of propagation
Frequency: number of waves to pass a given point per second and is also the number of complete vibrations of a
point on a wave (Hertz)
Wavelength: distance between two adjacent corresponding points of a wave e.g. between two crests or troughs or
two compressions or rarefactions
Velocity: the product of frequency and wavelength
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PHYSICS: THE WORLD COMMUNICATES 3
Describe the relationship between particle motion and the direction of energy propagation in transverse and longitudinal
waves
Transverse waves:
Particle motion: oscillates up and down
Direction of energy propagation: perpendicular to particle motion
Longitudinal waves:
Particle motion: vibrates back and forth
Direction of energy propagation: parallel to particle motion
Quantify the relationship between velocity, frequency and wavelength for a wave: v = f λ
Velocity of a wave is equal to frequency times wavelength
= where f is in Hz, is in metres and v is in ms-1
Perform a first-hand investigation to observe and gather information about the transmission of waves in:
- slinky springs
- water surfaces
- ropes
or using appropriate computer simulations
Present diagrammatic information about transverse and longitudinal waves, direction of particle movement and the direction
of propagation
Transverse waves:
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Longitudinal/Push waves:
Analyse information from displacement-time graphs for transverse wave motion
Perform a first-hand investigation to gather information to identify the relationship between the frequency and wavelength of
a sound wave travelling at constant velocity
A spring is used and the amount of time taken for a period to be completed is used to find frequency
A standing wave allows us to measure the wavelength
Frequency is inversely proportional to wavelength
As wavelength increases, the frequency decreases
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Task Outcome 2: Features of a wave model can be used to account for the properties of sound.
Identify that sound waves are vibrations or oscillations of particles in a medium
Sound waves are longitudinal mechanical waves
Requires a medium for propagation hence their particles vibrate back and forth
Relate compressions and rarefactions of sound waves to the crests and troughs of transverse waves used to represent them
Compressions are high pressure, rarefactions are low pressure
If pressure was graphed, compressions will be equivalent to crests, rarefactions will be equivalent to troughs
Explain qualitatively that pitch is related to frequency and volume to amplitude of sound waves
The higher the pitch, the greater the frequency of the sound wave
The lower the pitch, the lower the frequency of the sound wave
As the volume of a sound increases, the amplitude of the sound wave that created it also increases
Amplitude is the maximum displacement of a given molecule from its mean position hence the larger the
displacement, the greater the amount of energy required to produce it
Low Pitch and High Pitch Loud/Soft
Explain an echo as a reflection of a sound wave
The lower the pitch, the lower the frequency of the sound wave
An echo occurs due to the reflection of a sound wave from the surface of an object or material
As the sound wave reaches the end of its medium, it reflects and travels back towards the source
Incident wave bounces off the surface and the observer hears the reflection of the original sound some time
afterwards
Most effectively reflected from hard, smooth surfaces while soft, irregular surfaces absorb the most sound
Describe the principle of superposition and compare the resulting waves to the original waves in sound
The Principle of Superposition states that if two or more waves pass through the same medium at the same time the
displacement of any point is the sum of the individual displacement of each wave at that point
Superposition of waves result in a new amplitude only, the frequency is not affected
Waves out of phase: amplitude of produced wave is less than either of the original waves
Waves in phase: amplitude of the produced wave is greater than either of the original waves
Perform a first-hand investigation and gather information to analyse sound waves from a variety of sources using the Cathode
Ray Oscilloscope (CRO) or an alternate computer technology
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Perform a first-hand investigation, gather, process and present information using a CRO or computer to demonstrate the
principle of superposition for two waves travelling in the same medium
Present graphical information, solve problems and analyse information involving superposition of sound waves
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PHYSICS: THE WORLD COMMUNICATES 7
Task Outcome 3: Recent technological developments have allowed greater use of the electromagnetic spectrum.
Describe electromagnetic waves in terms of their speed in space and their lack of requirement of a medium for propagation
Electromagnetic waves are waves that do not require a medium to propagate
They propagate through changing and interacting electric and magnetic fields that are perpendicular to each other
They travel at the speed of light 3.00 x 108 ms-1
Identify the electromagnetic wavebands filtered out by the atmosphere, especially UV, X-Rays and gamma rays
Gamma rays: absorbed by the thermosphere
X-Rays: absorbed by the thermosphere
UV Rays: atomic oxygen and molecular nitrogen absorb high energy UV in the thermosphere while ozone absorbs
low energy UV in the stratosphere
Visible light: not filtered by the atmosphere
Infra-Red: mostly absorbed by the atmospheric gases
Microwaves: pass through the atmosphere
Radio waves: short wavelength radio waves pass through the atmosphere while long wavelength radio waves are
filtered out
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Identify methods for the detection of various wavebands in the electromagnetic spectrum
Gamma wave: Geiger counters, thermoluminescent detectors, X-ray film
X-Rays: X-ray film, electronic detectors and counters, Geiger counters
UV Rays: certain crystals that fluoresce under UV light, electronic photo-detectors, photomultipliers
Visible light: photoreceptors in eyes, electronic photo-detectors, light meters, photographic film, photomultipliers
Infra-Red: thermoreceptors in skin, thermocouples, electronic photo-detectors
Microwave: mobile phones, TV and satellite antennas, materials that fluoresce when exposed to microwaves
Radio wave: TV and radio aerials and antennas
Explain that the relationship between the intensity of electromagnetic radiation and distance from a source is an example of
the inverse square law: ∝1
2
Intensity is a measure of the amount of energy per unit of area
Intensity of a wave will decrease as you move away from the source
Light emanating from a point source spreads out in all d irections and travels at a constant speed, the energy of the
light will spread out in spheres
∝1
2 is the intensity of a uniformly transmitted wave with no mechanical energy loss decreases with the square
of the distance d from the source
Electromagnetic waves do not require a medium to propagate and in air there are practically no energy losses
Outline how the modulation of amplitude and frequency of visible light, microwaves and/or radio waves can be used to
transmit information
Modulation is the process of changing characteristics of a wave to add a signal to the carrier wave allowing useful
information to be transmitted
By altering either frequency, amplitude or phase of a carrier wave, the wave can act as a type of “code” to be
decoded and used
Information is added on a carrier wave by superimposing signals of varying frequency or signals of varying amplitude
but phase is rarely used
FM: frequency modulation, AM: amplitude modulation
Bandwidth: the range in which a modulating signal is limited to a narrow band of frequencies which is on either side
of the carrier frequency
Modulation of radio waves
Information broadcasted need to be limited to a band of frequencies ranging from 20Hz to 20000Hz
Each station is given a carrier wave of a particular broadcast frequency (tuning frequency)
A modulating signal is superimposed onto an unmodulated carrier of a certain frequency to produce a modulated
carrier
Modulating signal: the wave that contains the information to be sent
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Unmodulated carrier: wave that has the frequency it is transmitted at
Amplitude/frequency modulation: information required in the modulating signal is encoded in the
amplitude/frequency of the carrier wave
Demodulation: when the receiver receives the signal and subtracts the carrier wave from the modulated signal
Discuss problems produced by the limited range of the electromagnetic spectrum available for communication purposes
These restrictions are not placed on technologies which use optical fibres
Small bandwidth:
Small bandwidth in the EM range that can be used to effectively transmit signals
Causes congestion of frequencies leading to interference or no possible frequency which to transmit
As more and more people use these services, they will become more and more congested until the communication
network can no longer be supported by the narrow bandwidth
Health concerns:
Non-ionising EM poses significant health risks
Microwave radiation used in mobile phones is thought to create brain tumours and cause cancer
Perform a first-hand investigation and gather information to model the inverse square law for light intensity and distance from
the source
A light meter is used to record the intensity of light from various distances from a light source
A intensity vs. distance graph is constructed: very steep exponential relationship, possible intensity proportional to
some power of the distance
A intensity vs.1
2 graph is constructed: inverse exponential relationship, almost a positive linear function
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Identify the waves involved in the transfer of energy that occurs during the use of one of the following:
- mobile phone
- television
- radar
Mobile phone:
Sound wave (microphone) electrical energy (mobile phone) EM radiation (radio waves)(receiving cell tower)
electrical energy light (optical fibre)
Process continues in reverse to the receiver
Identify the electromagnetic spectrum range utilised in modern communication technologies
Communication: radio waves, microwaves, infra-red, visible light and UV
Radio waves: television, FM and AM radio, radar, some mobile telephone signals
Infra-red: telecommunications through optical fibres
Visible light: communication in fibre optic telecommunications and smoke signals
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Task Outcome 4: Many communication technologies use applications of reflection and refraction of electromagnetic waves.
Describe and apply the law of reflection and explain the effects of reflection from a plane surface on waves
Reflection is a wave property and can be described as waves bouncing off a surface, and thus is when a wave strikes
a boundary and is cast back into the medium in which it was originally travelling
Law of reflection: when waves reflect from a surface, the angle of incidence (i) is equal to the angle of reflection (r)
When waves are reflected, incident and reflected waves have the same frequency, wavelength and speed
Incident ray, reflected ray and normal all lie in the same plane
Angle of incident and angle of reflection are measured from the normal
Describe ways in which applications of reflection of light, radio waves and microwaves have assisted in information transfer
Reflection of light: fibre optics and CDs
Fibre optics allow massive amounts of information transfer over long distances
Reflection of radio waves: reflected off the ionosphere and used by television and radio
Describe one application of reflection for each of the following
- plane surfaces
- concave surfaces
- convex surfaces
- radio waves being reflected by the ionosphere
Plane surfaces:
Plane mirrors are a flat glass sheet over a silver backing and are highly reflective smooth surfaces
Forms images of objects that are in front of the mirror
e.g. CDs where laser beams are either reflected or not
Concave surfaces:
Concave mirrors magnify an image when the object is inside the focal point
Useful when trying to look at an object in greater detail e.g. dentists, make up mirrors
Reflects all focal rays parallel to the principal axes, allowing for a directed beam of light e.g. torches, car headlights
Convex surfaces:
Convex mirrors create virtual images diminished in size
Gives a wider field of vision e.g. driving mirrors, shopping centres, car rear-view mirror, carparks
Parallel rays incident on their surface reflect and diverge from a focus behind the mirror
Radio waves being reflected by the ionosphere:
Ionosphere: layer of ionised air approximately 50km to 640km above Earth’s surface and is ionised by UV radiation
Can absorb, reflect or allow some radio waves coming from earth to pass through
Low frequency or long wavelength waves reflect well Radio waves like light travel in straight lines and bounce off the small curvature of the atmosphere
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Higher frequency waves require a “line of sight” relationship between transmitter and receiver or they can be sent
to communication satellites and are relayed to another point such that multiple reflections between ground and
satellite stations allow the signal to travel around earth
Explain that refraction is related to the velocities of a wave in different media and outline how this may result in the bending of
a wavefront
Refraction is a wave property which describes the change in speed which occurs when light passes between any two
different materials of different densities
During refraction: frequency is constant, wavelength changes, velocity changes
Angle of incidence (i) is defined as the angle between the incident ray and the normal
Angle of refraction (r) is defined as the angle between the refracted ray and the normal
When waves travel into a relatively denser medium: ray bends towards the normal, i > r and wavelength and
velocity becomes smaller (slower)
When waves travel into a relatively less dense medium: ray bends away from the normal, i < r and wavelength and
velocity becomes greater (faster)
Medium 2 is denser than Medium 1 Medium 2 is less dense than Medium 1
Define refractive index in terms of changes in the velocity of a wave in passing from one medium to another
Relative refractive index of a medium: measure of how much velocity of light will change as it passes from one
medium to another
=
as the light ray goes from a to b ( ↑ = lower light speed, ↓ = higher light speed)
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Define Snell’s Law:v
v2=
sini
sinr
Snell’s Law states the ratio between the sine of the angle of incidence and the sine of the angle of refraction is a
constant, and is equal to the ratios of the velocity of light through the media
=
=
=
=
sin
sin where is the relative refractive index
Identify the conditions necessary for total internal reflection with reference to the critical angle
Critical angle () is the angle of incidence for which the angle of refraction is 90°
Critical angle only exists going from a more dense medium to a less dense medium
=
sin
sin=
sin
sin90° ∴ =
If the angle of incidence is greater than the critical angle (i > ic) then the wave will be “totally internally reflected”
Outline how total internal reflection is used in optical fibres
Total internal reflection describes the behaviour of light when it is reflected without refraction
Occurs when the angle of incidence of light as it is going from a denser medium into a less dense medium exceeds
the critical angle
Fibre optic cables are thin flexible rods that transfer information via light waves
Consist of two concentric layers of ultra pure, bubble free glass (fibre core and cladding)
Fibre core has the highest refractive index, cladding with a relatively lower reflective index, sheath protects the
inner layers to ensure no unwanted light enters the fibre and disrupts the signal
The refractive index of the fibre core must be greater than the cladding for total internal reflection to occur
Used for communication, doctor endoscopes and surgeons
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Perform first-hand investigations and gather information to observe the path of light rays and construct diagrams indicating
both the direction of travel of the light rays and a wave front
Use ray diagrams to show the path of waves reflected from:
- plane surfaces
-
concave surfaces
- convex surfaces
- the ionosphere
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Perform an investigation and gather information to graph the angle of incidence and refraction for light encountering a
medium change showing the relationship between these angles
Perform a first-hand investigation and gather information to calculate the refractive index of glass or Perspex
A light box is used to produce a narrow beam of light and directed into a Perspex block
Rays are traced and angles of incidence and refractions are measured
A sine of refraction vs. sine of incidence graph is constructed
A linear relationship is shown, with the gradient of the line of best fit showing1
Sin r is proportional to sin i
Refractive index of Perspex is 1.4
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Task 5: Electromagnetic waves have potential for future communication technologies and data storage technologies.
Identify types of communication data that are stored or transmitted in digital form
Digital form: data represented using binary code, a series of 1s and 0s
Uses: fax, internet, phone calls, text, picture, sound
Discuss some of the underlying physical properties used in one application of physics related to waves such as:
- Global Positioning System
- CD technology
- the internet (digital process)
- DVD technology
CD technology:
Compact discs are hard plastic disc which are metal coated then plastic coated
Data is stored in a continuous groove starting at the centre of the disc and spiraling outwards
An infrared recording laser of wavelength 780nm is focused onto the master disk and records digitized information
on the disk as a series of pits or small holes
“1” is represented by a deeper square pit while “0” is represented by lack of pits
Pits in the groove are about 0.5μm wide and up to 3μm long, separated by 1.6μm
When a CD is played, it rotates extremely quickly and a laser beam scans the surface of the disc so that the light is
either reflected from the metal surface or scattered by a pit
Optical sensors detect the light pattern produced and convert this to tiny digital pulses of electric current
Digital data is then converted to an analogue signal to produce pictures, videos and sound