Download Physics – A World Communicates

Physics – A World Communicates
Sec 1: The wave model can be used to explain how current technologies transfer information.
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Describe the
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energy transformations required in one of the following:
Mobile phone
Fax/Modem
Radio and Television
Mobile Phone
Mobile phones have a built-in microphone that changes sound waves into electrical signals. These electrical
signals are digitised and transmitted as radio waves to the base station.
The base station consists of a system of antennae on top of a tower or tall building. Each base station accepts
and can transmit radio signals from three adjacent hexagonal- shaped areas called cells.
Each base station is connected to a switching centre that carries the signal as electrical impulses. The
impulses have been produced by radio wave energy interacting with the aerial.
There are three main possible paths for the signal to take:
1) If the telephone call occurs between a mobile and a distant fixed telephone, the signal may be
converted into light and travel along a fibre optic network to a distant switching centre that is closer to the
fixed phone’s destination.
2) If a telephone call occurs between a mobile and a fixed telephone close to aerial, it is converted
into an electrical impulse in a copper wire. It may remain as an electrical impulse in the copper wire network
until it reaches a switching station closer to its destination. In this and previous case, the signal is then routed
along copper wire as an electric impulse and the telephone then converts it back into sound energy.
3) If the telephone call is from one mobile to another mobile, the signal from the switching centre
will be transferred to a switching centre close to a base station servicing the cell near where the receiving
mobile telephone is located. The signal is then fed to a base station as an electric impulse, and broadcast as
radio waves to the mobile. Once at the mobile the radio signal is converted into electrical signals and then into
sound energy.
In summary:
The following conversions take place within a mobile phone:
1) Energy is stored as chemical energy in the phones battery
2) Chemical energy is transferred into electrical energy to operate the phone
3) The microphone converts sound energy into electrical energy
4) The antennae converts electrical energy into electromagnetic energy and incoming electromagnetic
energy into electrical energy
5) The earphone/speaker converts electrical into sound
6) The LCD screen convents electrical into light.

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
The source of all waves is vibration. The energy of that vibration passing from place of origin to a place further
away is the wave.
For all wave motions, whatever their origin, the transfer of energy is in the direction that the wave is
travelling (i.e. in the direction of propagation). That transfer of energy is always from the source of the
vibration. When the source of the waves is from a point acting a source of vibration (known as the point
source) the waves radiate out from that point.
An example of a wave travelling in one dimension is the motion of either a transverse or longitudinal wave in
a slinky. In this case the medium confines the waves to the rope or slinky. The energy of the wave motion as
only one dimension to travel.
An example of a wave travelling in two dimensions is a transverse wave travelling from a point source of
disturbance in a still water. A pebble thrown in a still pint will produce a wave travelling outwards with a
circular wavefront away from the initial disturbance.
As an example of a wave travelling in three dimensions consider a point source of sound – it results in a
sound wave that immediately travels away from the source in three dimensions with a spherical wavefront.
Similarly, a point source of light will illuminate a three dimensional space.
For waves, the disturbance travels through the medium, but the medium doesn’t move forward with the
disturbance. It is the disturbance that travels through each of the materials that forms the waves. Waves pass
from one place to another without taking materials with them. They are simply a means of transferring energy
from place to place.
Consider:
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Sun’s light rays required for photosynthesis and thus life itself
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Reflection of light off objects onto our eyes results in vision. Different colours have different energies
and thus result in different stimulations in the brain.
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Microwaves used to cook food, by causing particles to vibrate and thus generating heat through friction.
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Earthquakes transfer stored energy from the rocks as a ‘shake’ causing destruction.

Identify that mechanical waves require a medium of propagation while electromagnetic
waves do not.
Waves are categorised according to how they propagate or transfer energy from place to place. There are two
major groups of waves: mechanical waves and electromagnetic waves
Mechanical waves involve the transfer of energy through a medium by the motion of particles of the
medium itself. The particles move as oscillations or vibrations around a fixed point. After the wave has
passed, the particles that move are in exactly the same place as before they were disturbed. There is no bulk
transfer of particles from one place to another.
Electromagnetic waves such as radio and light are able to propagate through the near vacuum of space
without a material that vibrates. These waves to not need the movement of any solid particles in order to
propagate and they are not subject to the same energy losses due to friction between particles such as
mechanical waves. Therefore, they potentially have much greater travel ranges, which is vital for long
distance communication.
Mechanical waves are classified as either transverse of longitudinal according to the direction of disturbance
or vibration relative to the direction of energy flow through the material.
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In a transverse wave, the particles of the medium vibrate in a plane that is perpendicular to the
direction of propagation of the wave
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In a longitudinal wave, the particles of the medium vibrate in the same direction of propagation of the
wave.
Note: Because mechanical waves transfer energy as an oscillation of particles it follows that there
must be a material substance to act as a medium for the transmission of a mechanical wave.
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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.
A sine wave is the general outline of the cross section of a wave as shown below:
Medium: Is the material in which the wave is propagating. Electromagnetic waves do not require a medium
Displacement: Is the distance that the particle is from its point of equilibrium
Amplitude: Is the maximum size of the particle displacement from its undisturbed state (equilibrium).
Period: Is the time it takes a single wave to pass a fixed point. It is assigned with the symbol, T.
Compression: Are regions of high pressure (applies to both transverse and longitudinal waves).
Rarefaction: Are regions of low pressure (applies to both transverse and longitudinal waves).
Crest: Is the highest point of the wave
Trough: Is the lowest point of the wave
Transverse Waves: This is when the particle motion is perpendicular to the propagation direction.
Longitudinal Waves: This is when the particle motion is parallel to the propagation direction.
Frequency: Is the number of waves that pass a fixed point per second. The frequency is assigned the
symbol, f, when used in equations. The frequency of waves is usually measured in cycles per
second, or hertz (Hz). One hertz is one cycle or wavelength passing a point per second.
Wavelength: The distance between two adjacent crests or troughs in a wave and is assigned the symbol, ( )
when used in equations.
Velocity: The velocity at which a wave propagates is the how fast the wave transfers energy away from a
source. If the wavelength and frequency of a wave are known then we can calculate the velocity.
Velocity is the product of frequency and wavelength that is:
Phase: Two points are on the same phase, it at any particular instant, they have the same displacement and
the same velocity.
Wavefront: This is either the crest or trough of a wave. The Wavefront is perpendicular to the direction of the
waves velocity.
Note: the Period and the frequency are related through a reciprocal relationship

quantify the relationship between velocity, frequency and wavelength for a wave:
λ f= v
The above equation states that the wavelength ( λ ) multiplied by the frequency ( f ) is equal to the velocity
( v ).
Note: the speed of sound in air is approximately 340 m/s

Describe the relationship between particle motion and the direction of energy propagation
in transverse and longitudinal waves
Transverse wave
The vibrational component of a transverse wave involves particles undergoing a motion perpendicular to the
direction of propagation.
Transverse waves can be modeled by a slinky. The wave (if continuous jerking of slinky is applied) appears to
be standing sill is called a standing wave. The motion of the spring coils is at ninety degrees to the direction of
wave travel and is about their equilibrium position. The energy pulse of the wave propagates forward.
Electromagnetic waves are transverse waves because they consist of alternating electric and magnetic force
fields at ninety degrees to one another and the direction of energy propagation.
Longitudinal Wave
Sound is one of the most common longitudinal waves. It cannot be seen, but its vibration can be felt by
touching a speaker. Modelling a longitudinal wave can also be done with a slinky. The pulse that is produced in
the spring acts like a longitudinal wave. The compression, or wave pulse, travels along the length of the
spring. On each side of the compressed pulse is a zone where the slinky coils are spread apart. These parts
are known as rarefactions. The motion of the particles produced by the moving energy of the wave motion
is back and forth in the direction of the wave propagation.
In the case of sound waves, the pulses are compressed air particles surrounded on either side by particles
that are spread out. Compressions are zones of high pressure, while rarefactions are areas of low air pressure.
The amplitude of longitudinal waves is equal to the size of the maximum displacement of the particles from
their equilibrium position.
Sec 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
All sound waves are vibrations in a medium that result in pressure variations within that medium. The origin
of a sound wave in any medium is always a vibration. The frequency of the original vibration determines the
frequency of the sound produced by that vibrating object. The higher the pitch of the sound, the greater the
rate of vibration of the object. The objects vibration transfers some of its energy of movement to the medium
that carries the sound wave or vibrational energy.
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Relate compressions and rarefactions of sound waves to the crests and troughs of transverse
waves used to represent them
A compression is a zone where the particles of the medium are pushed closer together. It is a zone of higher
pressure.
A rarefaction is a zone where the particles of the medium are spread further apart. It is a zone of low
pressure.
The Cathode-Ray oscilloscope (CRO) is a device that allows us to view sound waves on a screen.
The areas where the displacement of the wave is above the base line represents zones of compression.
The areas where the displacement of the wave is below the base line represents zones of rarefaction.
The base line represents silence in a normal ‘sine curve’ wave. A trace showing only the base line would
indicate no sound at all.
The relationship between crests and troughs with compressions and rarefactions is shown below.

Explain quantitatively that pitch is related to frequency and volume to amplitude of sound
waves
Amplitude is the maximum size of the particle displacement from the undisturbed state. In relation to sound
waves, the higher the amplitude, the louder the sound.
When sounds are brought near a microphone, the louder sound will generate a higher voltage
(electrical energy) and thus will be represented with a greater amplitude. When replayed back, the
higher the amplitude the louder the sound.
The pitch of a sound is directly related to its frequency. The higher the frequency of the sound, the more
vibrations per second and the higher the pitch. A low frequency sound is a low pitched sound.

Explain an echo as a reflection of a sound wave
An echo is a repeated sound created by the reflection of sound waves from a surface.
When an echo bounces back from a solid surface the entire sound is not heard but the last part is. If you are a
significant distance from the surface, you will hear more of the original sound bounce back. If you are close to
the reflecting surface, you probably won’t detect an echo. It does still occur but the original sound drowns it
out.
There needs to be a time difference between the reflected sound and the original sound so that you can hear
the echo. The size of that time difference is a minimum 0.1 seconds. Since sound travels at 340m/s, you must
be at least 17m from the surface reflecting the sound. At this distance, the sound wave takes 0.05 seconds to
reach the reflecting surface from the sound source and 0.05 seconds to bounce back.
Note: That the wave that is bounced back is out of phase with the wave hitting the wall. This phase
change upon reflection is a characteristic of all waves.
Echoes are used by sonar rangers to determine the distance to objects. This may be to test ocean depth, or to
see how much fuel is left in storage tanks in industry. In most of these applications it is desirable to use shortwavelength, high-pitched sound waves. These ultrasonic or high-frequency sound waves are emitted
from a source and bounced back. Sensors measure the time take for the echo to occur and thus the
distance can be measured (the speed is assumed to be constant despite the fact there may be slight
fluctuations due to changes in density of medium).
Furthermore:
Because there is a time difference between reflections of the same pulse if the reflecting surface is
irregular in shape, it is possible to use ultra-high-frequency sound waves to ‘see’ objects.
The reflections from multiple surfaces can be processed by a computer to generate an image of the body.

Describe the principle of superposition and compare the resulting waves to the original waves
in sound.
Sound waves from separate sources interfere with each other and can be added. When this occurs it is
possible to produce a sound of higher or lower amplitude depending on whether the sound waves are in or out
of phase.
Note: If the amplitude of the crest of one wave is precisely equal to the amplitude of the though of another
wave, but the second wave is out of phase by 180 degrees, the a complete loss of amplitude in the resulting
wave can occur. This means it is possible to add sounds together and produce no sound.
The addition of waves is known as superposition. The superposition principle states that if two or more
waves of the same type pass through the same medium at the same time, then the amplitudes of the wave
add together. The principle involves adding individual displacements at various points in a systematic way.
If two out of phase waves interfere, the amplitude of the resulting sound wave will be less than either the of
the original sound waves.
If two in phase sound waves appear, the resultant sound wave will have a greater amplitude than either of the
original waves.
Example of use:
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The development of technology has enabled the production of sound waves that are out of
phase by 180 degrees. This has been used to reduce the sound emitted by heavy machines in
factories.
Beats – A special case of superposition
The term beats refers to the change in volume when two sounds of slightly different frequencies occur
together.
When two sources of sound of the same amplitude but slightly different frequency are heard together, there
will be a rhythmic change to the volume of the sound. When the two sound waves are in phase, the amplitude
of the resulting sound wave is the sum of the amplitudes of the two waves and results in a large sound. As the
waves drift out of phase, the resultant amplitude becomes smaller, eventually reaching zero before increasing
again as the waves drift back into phase.
Extra Note: Timbre is the quality of a sound that depends on the way in which a number of different pure
sound have combined.
Sec 3: Recent technological developments have allowed greater use of the electromagnetic
spectrum

Describe electromagnetic waves in terms of their speed in space and lack of requirement of a
medium for propagation
Electromagnetic waves do not require a medium in order to travel from place to place. In fact, they travel
most efficiently in a vacuum, such as space. Electromagnetic waves travel at the speed of light and can
be reflected, refracted and can carry information as codes.
The electromagnetic spectrum is a continuum of electromagnetic waves with artificial (man made) divisions
based on the frequency and wavelengths of the waves.
There is no distinct point at which the frequency changes and no special change in properties at particular
wave boundaries.
Electromagnetic waves have special properties that are outlined below:
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All electromagnetic energy that passes through the vacuum of space at the common
speed of light (300 million m/s or 3 x 10^8 m/s)
The waves are produced by oscillating, perpendicular electric and magnetic fields, hence the
term electromagnetic
The waves are considered to be self propagating; that is, the electric field produces a
perpendicular magnetic field that in turn induces an electric field and so on. This property
enables them to travel immense distance for billions of years.
Electromagnetic waves are represented as sine waves. In general electromagnetic waves are
made up of multiple sine waves superimposed
They are capable of creating an electric response in the medium that they impact on
They have frequencies related directly to the vibrational frequency of the source
Can be refracted and reflected, when passing through mediums of different densities
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
Identify the electromagnetic wavebands filtered out by the atmosphere, especially UV, X-rays
and gamma rays
The Earth’s atmosphere and ionosphere absorb most of the incoming electromagnetic radiation from space
except for visible light and some high-frequency radio waves in the microwave region.
The other types of radiation would be generally be harmful to us. Too much UV radiation, for example, would
cause cancer and dangerous mutations. Too much penetrating by X-rays or gamma rays would quickly kill us.
The only wavebands that are allowed completely through the atmosphere are visible light,
microwaves and radio-waves.
The ionosphere as the filter.
A layer of gas called the atmosphere surrounds the Earth. At high altitude, roughly between 50km and 500km
above the Earth part of that gas is ionized; that is, by losing or gaining electrons the atoms and molecules of
the gas become charged. These ions form the layer of the atmosphere referred to as the ionosphere.
The ionosphere can be divided into three layers (D, E and F) based on the type of electromagnetic radiation
absorbed in each layer.
The regions of the ionosphere have the following characteristics:
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The D-region extends about 50-80km above the Earth’s surface. Hard X-Ray radiation with
short wavelengths and high frequencies are absorbed
The E-region extends about 80-145km above the Earth’s Surface. Soft X-rays absorbed
(longer wavelengths)
The F-region is from about 145-300km, Extreme UV radiation is absorbed in this region
Note: Due to solar flares, the ionisation of the stratosphere is increased (ie, there is a dramatic increase in
electromagnetic radiation), and this can cause fluctuations in the absorption and reflection capabilities of the
layers. For example the D region may absorb more lower frequency radio waves and this can lead to
interruptions in radio communication on Earth.
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Identify methods for the detection of various wavebands in the electromagnetic spectrum
Wave
Methods of Detection
Radio Waves
Resonant Circuit
Microwaves
Mobile phones, magnetrons, electric circuits
that are resonant with microwave frequency.
Infrared Waves
Skin, thermal camera
Light
Eye, photographic film, solar cells
UV Waves
Photographic film, photoelectric devices,
Chemical fluorescence
X-Rays
Photographic film, chemical fluorescence
Gamma Rays
Bubble chamber, Geiger counter
Sodium iodide (photo multiplier).

Analyse information to identify the electromagnetic spectrum range used in modern day
communication technologies
Type of Radiation
Communication Use
Limitations
Gamma Rays
Not Used
Dangerous, causes cancer
X-Ray
Not used
UV
Optical fibre
Visible
Optical fibre
Infra-red
Optical Fibre
Hand-held remote controls
Microwaves
Mobiles
Radio waves
Radio
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Difficult to generate using solid state
Electronics
Interference with ambience
Scattering – line of sight is used
Absorption by atmosphere
Attenuation occurs in optical fibres due
to absorption by impurities in glass
Line of sight
Line of sight
Interference from atmosphere
limited data carry due 2 low frequency
Discuss problems produced by the limited range of the electromagnetic spectrum
available for communication use.
The restricted range of frequencies available for communication use has caused governments to limit the
bandwidth over which certain communication devices can operate. There are a number of reasons for keeping
the major uses of bandwidth at a distance from each other on the spectrum, for example:
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to avoid problems with interference. Different technologies need different bandwidth
separations.
To provide equity for users. The communication industry is competitive with bandwidth the
main subject behind this
Enables communications to form part of the safety infrastructure.
Allows for new technologies to be developed that may require spectrum bandwidth.
Infra-red radiation and light are also used for communication over long distances via optical fibres. In this
case their use is not restricted by the range of spectrum available because they are utilized within enclosed
systems where penetration and attenuation are the issues that restrict the spectrum range used.
The greatest advantage of communication technology that uses electromagnetic waves is that communication
can be made extremely rapidly over long distance. The greatest problem is that the signals decrease in
strength the further they are from the source.
This decrease in the strength of the signal or light is known as attenuation. To reduce attenuation
effects over long distance communication the electromagnetic waves need to be either sent out as a very
large strength signal initially or the signals travelling large distances need to be amplified at amplifiers along
their route.
Further Note: Greater frequency means more information can be sent. Thus by limiting the range of
the electromagnetic spectrum, we are hindering the ability to send mass of data extremely rapidly.

Explain that the relationship between the intensity of electromagnetic radiation and
distance from a source is an example of the inverse square law:
Light and the Atmosphere
The intensity of visible light is affected by the atmosphere. White light is made up of all the colours of the
spectrum. The visible spectrum of electromagnetic radiation extends from red light, at around 700nm
wavelength, to violet, 400nm.
However, not all of that radiation is equally successful at penetrating the atmosphere.
Evidence for this uneven penetration is seen when the path that light must travel through the atmosphere is
at its longest; that is, at sunrise and sunset. Light must travel further and penetrate more of the atmosphere
at sunrise/sunset when the Sun is low – it is travelling ‘across’ the atmosphere not down through it as it would
around the middle of the day. The Sun looks red at sunrise/sunset. Red light is at low frequency, longer
wavelength whereas violet is high frequency, short wavelength.
Therefore, visible light of low frequency penetrates the atmosphere more successfully than the visible light of
high frequency.
Inverse Square Law
The relationship between distance from the source and strength or intensity of the electromagnetic wave
signal is clear. Increasing the distance between the receiver and the source results in a decreased intensity of
the received signal. The relationship between intensity drop off and distance from source is an example of the
inverse square law.
The inverse square law says that intensity of the signal varies inversely with the square of the distance. If the
receiver is double the distance from the source, the intensity is reduced to a quarter of the original intensity.
The easiest type of electromagnetic radiation to observe is light. Light intensity (illuminance) is measured in
units called lux (lx) using a light meter.
Using the inverse square law, if you were one meter from a light source where the intensity was 16000 lx,
then:
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At two metres the light intensity would be reduced by a factor of 4 – 4000 lx
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At three metres the light intensity would be reduced by a factor of 9 – 1777.8 lx
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At 4 metres the light intensity would be reduced by a factor of 16 – 1000 lx.

Outline how the modulation of amplitude or frequency of visible light, microwaves and / or
radio waves can be used to transmit information.
All waves carry energy from place to place. A wave that carries exactly the same amount of energy
continuously does not carry information. For a wave to carry information, it must vary.
The changes in amplitude and frequency of the sounds coming from an object have added information to the
signal by modulating the sound wave.
Electromagnetic waves can carry information but information must be added to the waves. The process of
adding the signal information to an electromagnetic wave is called modulation. Demodulation is the process
of converting the modulated signal back into usable form.
Bandwidth describes a series of adjacent frequencies forming a band within the spectrum.
Modulating a radio wave
A radio wave signal occupies a bandwidth of frequencies. This means that the transmitted electromagnetic
wave is using a number of frequencies that lie next to each other rather than a single one. In the middle of
the bandwidth is the carrier wave which does not carry any information. A signal wave and the carrier wave
are superimposed to produce the message signal.
Amplitude Modulation
Used in AM radio. The AM signal remains constant in frequency bandwidth but the amplitude of the wave
varies. The variation in the amplitude of the wave is decoded by a radio receiver to produce the signal which is
then converted into sound.
Frequency Modulation
In this the signal part of the wave has been added to the carrier wave to vary the frequency of the wave.
Frequency modulation is also accomplished with the superposition of a signal wave onto the carrier wave.
A limiting circuit in the radio receiver keeps the amplitude constant during the transmission and receiving. The
signal is converted back into sound by a discriminator circuit.
Advantages of FM:
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Noise is reduced by the limiting circuit in the receiver. This is because the FM radio signal is not
dependant on the strength (amplitude) of the signal received, but rather relies on the frequency
changes to provide the radio signal. It is much harder to change the frequency by interference,
hence the music received is closer to the actual broad cast.
Advantages of AM:
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Require less bandwidth of frequencies required for transmission, this allows fro a greater number of
transmissions than FM (i.e more radio stations).
Modulation of Microwaves
The principle of modulating the frequency of electromagnetic waves to carry a signal applies also to
microwaves used for transmitting mobile phones signals.
Microwaves are preferred over longer wave length radio waves for mobile telephone systems because:
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The electromagnetic spectrum is limited and the microwave bandwidth has the capacity
Microwaves do not spread out as rapidly as radio waves, allows for a greater proportion of
energy to be received.
Larger bandwidth than radio waves, and thus larger number of frequencies available for
frequency modulation – larger number of signals at once.
Microwaves require line of sight transmission. The microwave signals are diffracted by objects larger than
their wavelength. As a result reception in buildings is more difficult for shorter microwave wavelengths.
Microwave transmissions also have their range affected by atmospheric conditions such as the moisture
content of the air. Oxygen also absorbs microwave energy. Moisture and oxygen molecules absorb
microwaves in the atmosphere.
Modulation of Visible light
Light modulation is also used to carry signals. The signal for light must be amplitude modulated because the
frequency of light from a laser is of a fixed range too small for effective frequency modulation.
Short distance line of sight communication is possible using an amplitude modulated laser beam. Longer
distance transmission is less reliable because of the possibility of interference. For that reason, long distance
communication by shorter wave electromagnetic radiation such as infra-red radiation and light, is
accomplished along fibre optic cables to eliminate the chance of interference to the signal.

Analyse information to identify the waves involved in the transfer of energy that occurs during
the use of a radar.
Radar works by sending out evenly spaced pulses of radio-wave energy of a precisely-known wavelength.
Electrons moving in an alternating current that has precisely controlled frequency, move up and down in the
aerial to create these pulses. If the wave pulses strike an object, such as an aeroplane they reflect back,
creating an echo that the radar antenna can detect.
Usually the source of the radio waves generate an alternating electric current information about where the
radio wave echo is coming from, whether the wavelength has been altered and how long it takes each pulse to
return. It then creates a visible track of the moving object on the radar operator’s visual display.
Since radio waves travel at the speed of light, the radar must have an accurate high speed clock to use the
information it fathers. The radar can then measure the speed of an object using the Doppler effect.
The Doppler effect is as follows:
if the object is moving away from the source of the radio-waves, the wavelengths will be lengthened
as a result of collision and refraction
If the object is moving towards the source, the wavelength of the reflected radio is shortened.
The relative change in the wavelength is proportional to the speed of the object either towards of away from
the radar antennae.
Radar has applications that include: Speed detectors for police, tracking satellites, mapping Earths surface,
and tracking of Aeroplane traffic.
Sec 4: Many communication technologies use applications of reflection and refraction of
electromagnetic waves.

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 the phenomenon where waves, that are incident on any angle except the normal, bend as they
pass from one medium or depth to another.
Refraction can be seen in water
Refraction can be seen in surface where water changes depth.
If the incident water waves strike the shallow water region at an angle, the same slowing effect on the waves
occurs, but now the wavefronts have bent. The
The refraction of water waves is a characteristic of the behaviour of all waves. The difference in water depths
has the same effect as a change in the medium. A change in medium also affects the wave because each type
of wave has a fixed velocity in any given medium. The wave velocity changes when wave moves across an
interference from one medium into another. The frequency of the wave does not change. Therefore, from the
wave equation:
It means that the wavelength, ( ) must change.
The apparent bending of the object at the interface between air and water suggests that light entering water
is bent. Therefore it must have slowed down.
When a wave moves from one medium to another where its speed is lower, the ray bends towards the
normal.
When a wave moves from one medium to another where the speed is higher, the ray bends away from the
normal.


Define refractive index in terms of changes in velocity in passing from one medium to another
Define Snell’s Law
If a ray passes from a vacuum to another material of fixed composition and density, the degree of bending
which occurs, at the interface between the vacuum and the material is constant. This constant is given the
symbol n and is known as the refractive index This means that all refractive indexed are measured with
respect to a vacuum.
A vacuum by definition has an absolute refractive index of 1 for the electromagnetic spectrum. The absolute
refractive index of any transparent material is a measure, or ratio, of how much an electromagnetic wave
slows down at the interface between a vacuum and that material.
That is, the absolute refractive index of some material:
Snell’s Law applies equally to waves slowing down and speeding up as they move across the interface
between one medium and another. As with water waves at an interface, the frequency of the waves does not
change as they speed up or slow down, so it is the wavelength of the wave that changes.
This is expressed as Snell’s Law Relationship:
The refractive index is useful for determining what will happen to electromagnetic waves that pass across an
interface between transparent materials. The absolute refractive indices can be used directly to determine a
number of factors. This comes about because:

Identify the conditions necessary for total internal reflection with reference to the critical angle
Total internal reflection is a case of Snell’s Law in operation. It may occur when a ray of light attempts to
cross into a low-refractive index medium. The ray bends away from the normal in this circumstance.
Because a small change in the angle of incidence causes a larger change in the angle of refraction when the
situation shown below occurs, it is possible for an angle to of incidence to reach the critical angle where the
ray cannot exit the higher refractive index material. The ray is refracted so much that it is bent to 90 degrees
from the normal
The angle of incidence where the ray is just trapped in the higher refractive-index material is the critical
angle. If the critical angle of incidence of a ray at the interface of the two substances is exceeded the
interface will act as a mirror and total internal reflection of light rays occur. The ray then obeys the Law of
Reflection at the interface between the materials. The ray is trapped internally within the denser material.

Outline how total internal reflection is used in optical fibres
Total internal reflection has a number of practical uses. One of those is in optical fibres. This application of
total internal reflection of electromagnetic waves has led to optical fibres becoming major data carriers in
tele-communications.
Optical fibres are made from thin, cylindrical strands of ultra-high-purity glass. These optical fibres are made
so that they have a central, high-refractive-index region called a core. Their outer region, called the cladding,
is made from a lower refractive index glass. After electromagnetic radiation optical fibre it is totally internally
refracted at the interface between the higher-refractive index core and the lower refractive index cladding.
This means that rather than escaping through the surface of the optical fibre, the light is trapped internally
and continues to move forward in the optical fibre
Optical fibres are used in a number of situations, including:
Communication for carrying signals precisely and at the speed of light. Fastest form of
communication possible
Medicine. Operating doctors view sites such as the intestines, previously inaccessible without
invasive surgery.
Optical fibres are quite flexible which allows electromagnetic radiation, in particular, visible and infra-red
radiation to be reflected easily and precisely around corners without the need for any physical reflective
device.