Chapter 6 Waves 2011

8/6/2019 Chapter 6 Waves 2011

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Hoo Sze Yen www.physicsrox.com Physics SPM 2011

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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Hoo Sze Yen Physi

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

Chapter 6 Waves 2011

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