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CHAPTER 6:
WAVES
6.1 Wave Basics Waves are generated by oscillating/vibrating systems
An oscillation is the back-and-forth movement of an oscillating system through a fixedpath
6.1.1 Wave Fronts Wave fronts are the lines or surfaces connecting the particles moving at the same phase
and are at the same distance from a wave source.
Wave fronts are always perpendicular to the direction of propagation.
Plane waves
Circular waves
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6.1.2 Typ es of WavesTransverse Waves Longitudinal Waves
Transverse waves are waves which oscillate
perpendicular to the direction of
propagation.
Longitudinal waves are waves which
oscillate parallel to the direction of
propagation.E.g: Light waves, Water waves E.g: Sound waves
6.1.3 Amplitude , Period and Frequenc y Amplitude is the maximum displacement of an object from its equilibrium position [m]
Period is the time taken for a particle to make one complete oscillation [s]
nsoscillatioofnumber
takentime
Period,=
T
Frequency is the number ofcomplete oscillations in one second [Hz]
takentime
nsoscillatioofnumberFrequency, =f
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A system that is forced to oscillate continuously with provided external energy is said tobe undergoing forced oscillation
Natural frequency is the frequency of a system that is left to oscillate freely without anexternal force
An object that is forced to oscillate at its natural frequency is said to be vibrating atresonance. An object vibrating at resonance has the maximum amplitude because it is
receiving maximum energy from the external system
Bartons Pendulum
When the control pendulum X is oscillated, its energy is transferred to the otherpendulums through the string.
The other pendulums are forced to oscillate at the same frequency as pendulumX.
Because pendulum D has the same natural frequency as X (same length), pendulum Dwill oscillate at resonance and will have the maximum amplitude.
6.1.7 Ripp le tankAll water wave phenomena are observed through ripple tanks.
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Formation of wave shadows on the screen
6.2 Wave Reflection6.2.1 Reflec tion of Waves
The angle of incidence = The angle of reflection
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6.2.2 Patterns of wa ter wave reflec tionsPlane waves Circular waves
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6.2.3 Applications Embankments to protect the ports, beaches, etc
6.3 Wave Refraction6.3.1 Water wa ve refrac tion Water travels faster in deep waters and slower in shallow waters
Therefore, the wavelength of water waves in deep water is bigger than the wavelength inshallow water.
1 > 2
When traveling from deep to shallow, the waves refract towards normal
When traveling from shallow to deep, the waves refract away from normal
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6.3.2 Water wa ve refrac tion p atterns
6.3.3 Water wave refrac tion at the seaside As the wind blows the sea towards the
beach, the decreasing depth causes the
speed of the water waves to slow down
The refraction effect causes the wavefronts to curve to be almost parallel to the
beach
In the middle of the sea, the wavefronts are almost in a straight line, as
perA1B1C1D1 due to the same waterdepths
As the waves approach the beachline,the wave fronts begin to curve to
follow the shape of the beachline, as
perA2B2C2D2 andA3B3C3D3 Energy from A1B1 is focused on the
peninsula atA3B3 causing thepeninsula to be hit by strong waves
Energy fromB1C1 is spread out through the bay atB3C3 causing the water at the bay to becalmer
6.3.4Sound wave refrac tion
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6.4 Wave Diffrac tion6.4.1
Wave diffrac tion
Diffraction is more visible when: The wavelength of the wave is bigger The obstacle is smaller than the wavelength The aperture is smaller than the wavelength
Smaller apertureDiffraction is more obvious
Bigger apertureDiffraction is less obvious
Smaller obstacle
Diffraction is more obvious
Bigger obstacle
Diffraction is less obvious
Round obstacle
6.4.2 Applications of d iffrac tion Embankment to protect ports
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6.5 Wave Interference6.5.1
Principle of superposition
The principle of superposition state that when two waves propagate through the same point at the same time, the displacement at that point is the vector sum of the
displacement of each individual wave.
Two wave sources which are coherent have the same frequency and the same phase orphase difference.
The superposition effects creates interference
Constructive interference Destructive interference
6.5.2 Interference pa ttern
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6.5.3 Interference eq uation
D
ax=
where = wavelength [m]
a = distance between sources [m]
x = distance between two successive antinodal/nodal lines [m]D = distance between a andx [m]
6.5.4 Different frequenc ies
Low frequency
(large wavelength)
High frequency
(small wavelength)
Value of x is larger Value of x is smaller
6.5.5 Different distance between the sources
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6.6 Sound Waves Sound waves are longitudinal waves.
Sound waves are mechanical waves; therefore they need a medium to propagate.
The medium undergoes compression and rarefaction to transfer the energy of the soundwaves from one point to another.
6.6.1 Spe ed of sound Speed of sound is fastest in solids, followed by liquids, then gases.
Speed of sound increases with temperature
6.6.2 Amplitude and Loudness The loudness of sound is dependent on the
amplitude of the wave.
The higher the amplitude, the louder thesound.
6.6.3 Frequency and Pitch The pitch of sound is dependent on the
frequency of the wave.
The higher the frequency, the higher the pitch.
6.6.4 Qua lity of Sound Different musical instruments can produce notes of the
same loudness and pitch, and yet they are easily
discernible from one another.
This is because of the quality or timbre of the noteproduced by the individual musical instruments.
Quality of sound depends on the shape of the soundwaves generated by the musical instruments.
Each note consists of a fundamental frequency that ismixed with weaker frequencies called overtones.
6.6.5 Frequenc y rangesInfrasonic / Subsonic Frequency too low for human ears Below 20 Hz
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6.6.7 App lic ation of sound wave phenom ena Echoes (Sound wave reflection) In an auditorium, concert hall or music studio, echoes
must be taken into account to ensure good acoustics
Hyperbolic shape of sound waves Ampitheatres are usually designed in a hyperbole to
enable better sound travel
Sonar Supersonic waves used to measure the ocean
depths and to detect objects in the ocean The transmitter releases an ultrasonic pulsewhich echoes off the ocean bed or object and
is detected by a hydrophone
Ultrasonic waves in medicine Diagnostics to create a picture or an image of an internal
organ. E.g. foetus in mothers womb
Ultrasonic drill to cut a decaying part of the tooth
Ultrasonic waves in industries Ultrasonic echoes to detect flaws in a metal structure.
E.g. in railway tracks
Ultrasonic drill to cut holes in glass and steel High frequency vibration to clean instruments and fragile items
6.7 Elec tromag netic Waves Electromagnetic waves are electrical and
magnetic fields oscillating perpendicular
to each other around a single axis
6.7.1 CharacteristicsElectromagnetic waves have the following
characteristics:
Transverse wave
Fulfills the wave equation v=f
Travels at the same speed (speed through vacuum: c = 3 108 m s-1)
Does not need a medium to propagate
Can be polarized
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6.7.2 Elec trom agne tic Wave Spec trumElectromagnetic wave Source Characteristic Uses
Gamma ray Nuclear
reaction(fission,
fusion)
High energy
High penetration Extremely
dangerous
Kill cancer cells
Sterilization Food preservation
Kill agricultural pests
Detect flaws or wornparts in car engines
X-ray X-ray tubes:high-velocity
electrons
hitting heavymetal targets
High energy
High penetration
Extremely
dangerous
Detect bone flaws orfractures
Detect structural or
machine flaws Investigate crystal
structures and
elements in a material
Examine bags at theairport
Ultraviolet
ray The sun
Mercuryvapour lamps
Extremelyhot objects
Absorbed byglass and the
ozone layer
Enables chemicalreactions, skin
burns, skin cancer
Treats the skin withthe right exposure (for
Vitamin D)
Detects counterfeitmoney
Visible light The sun
Light bulbs
Fire
Consists of sevencolours with their
own respective
wavelengths and
frequencies
Enables vision
Enables photography
Photosynthesis
Optic fibre to seeinside tissues and
organs Laser light in optic
fibre forcommunication
Infrared ray The sun
Heater
Hot orburning items
Heat ray
Enables a hotfeeling
Physiotherapy
Pictures of internalorgans
Satellite pictures
Microwave
Klystroms
Penetrates theatmosphere
Communication satellite, radar
Cooking
WAVELENGTH,
(m)
Radiowave
UHF
VHF
SW
FREQUENCY,
f(Hz)
Electricalcurrents
oscillating at
th
VHF & UHF
Radio and televisionSW, MW & LW
R di b d t
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Chapter 6: Waves Page 15 of 15
6.8 Wave Phenom enaPhenomena Changing
characteristics
Water waves Sound waves Light waves
Reflection Unchanged:
Speed
Frequency
Wavelength
Change:
Amplitude
Refraction Unchanged:
Frequency
Change:
Speed
Wavelength
Amplitude
Carbon dioxide: Converges thesound waves (louder)
Helium: Diverges the sound waves
(softer)Diffraction Unchanged:
Speed
Frequency
Wavelength
Change:
Amplitude
Results using single-slit slide:
Interference Unchanged:
Speed Frequency
Wavelength
Change:
Amplitude
Results using Young double-slit:
normalIncidentray Reflectedray
Ra box Slide Screen
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