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Chapter 1: Chapter 1: WavesWaves

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The study of The study of mattermatter

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Objectives: Objectives: ((what you will learnwhat you will learn)) 1) understanding Waves

2) reflection of waves

3) refraction of waves

4) diffraction of waves

5) interference of waves

6) analysing sound waves

7) analysing electromagnetic waves

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Understanding Waves:Understanding Waves:

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1.1. A wave is a traveling A wave is a traveling disturbancedisturbance from a vibrating or oscillating from a vibrating or oscillating source.source.

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2.2. A wave carries energy along with it A wave carries energy along with it in the in the directiondirection of its propagation. of its propagation.

3.3. A wave is a mean of A wave is a mean of energy energy transfertransfer through vibration. through vibration.

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Transverse Wave

WavesWaves

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Particles in the medium vibrate in a Particles in the medium vibrate in a direction direction perpendicularperpendicular to the to the direction of wave propagation.direction of wave propagation.

PerpendicularPerpendicular= 90= 90oo to the to theline of direction.line of direction.

ExamplesExamples::wave in a ropewave in a ropewater waveswater waveslight waveslight waves

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Particles in the medium vibrate in a Particles in the medium vibrate in a direction direction parallelparallel (0 (0oo to line of to line of direction) to the direction of wave direction) to the direction of wave propagation.propagation.

ExamplesExamples::wave in a slinky springwave in a slinky springsound wavesound wave

WavesWavesLongitudinal WaveLongitudinal Wave

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WavefrontWavefrontA A surfacesurface on the wave where all particles on the wave where all particles vibrate in vibrate in phasephase (coming together to the (coming together to the same level).same level).OscillationsOscillationsVibration or oscillation of particles in a Vibration or oscillation of particles in a medium is like oscillation of simple pendulum medium is like oscillation of simple pendulum or loaded spring.or loaded spring.

WavesWavesSurface WavesSurface WavesThis is both transverse waves & This is both transverse waves & longitudinal waves longitudinal waves mixedmixed in one in one medium.medium.

ExamplesExamples::earthquake or seismic waveearthquake or seismic waveshear wave in a slinky springshear wave in a slinky spring

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WavesWavesComplete OscillationComplete OscillationComplete cycle; Complete cycle; e.g. motion from A to B & back to A.e.g. motion from A to B & back to A.

Amplitude, aAmplitude, aMaximum displacementMaximum displacement from equilibrium from equilibrium position that is halfway between crest (high) position that is halfway between crest (high) & trough (low).& trough (low).Period, TPeriod, TTime taken for a complete oscillation.Time taken for a complete oscillation.

Frequency, fFrequency, fNumber of complete oscillation in one Number of complete oscillation in one second;second;

f = 1/Tf = 1/T

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WavesWavesSpeed of wave, vSpeed of wave, vDistance Distance traveledtraveled by wave per second, v = by wave per second, v = ffλλFree OscillationFree OscillationOccurs when a system oscillates Occurs when a system oscillates withoutwithout any any external force acting on it.external force acting on it.

Natural Frequency, Natural Frequency, ffnnFrequencyFrequency of a free oscillation. of a free oscillation.

Simple pendulumSimple pendulum

ll = length, = length, gg = = gravitygravity

Loaded vertical Loaded vertical springspring

mm = mass, = mass, kk = = constantconstant

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WavesWavesDamped OscillationDamped OscillationAn oscillation whose An oscillation whose amplitude decreasesamplitude decreases due to due to energy lossenergy loss from the system (as heat from the system (as heat to air resistance).to air resistance).

Forced OscillationForced OscillationAn oscillation produced when a system is An oscillation produced when a system is forcedforced into oscillation by an into oscillation by an externalexternal periodic force.periodic force.ResonanceResonanceOccurs when a system which is forced into Occurs when a system which is forced into oscillation oscillates with oscillation oscillates with large amplitudelarge amplitude; ; the condition being that the frequencies of the condition being that the frequencies of both systems are both systems are equalequal to each other. to each other.

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Reflection of Reflection of WavesWavesAngle of reflection = angle of Angle of reflection = angle of

incidenceincidenceReflected wave has same speed, frequency, Reflected wave has same speed, frequency, & wavelength as the incident wave.& wavelength as the incident wave.

λλ λλ

ii rr

vv vv

Angle of reflectionAngle of reflection, r = , r = Angle of incidenceAngle of incidence, i, i

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Refraction of Refraction of WavesWavesRefraction of waveRefraction of wave

The change in The change in velocityvelocity of wave when it of wave when it travels from one medium to another.travels from one medium to another.

Water & light waves are refracted Water & light waves are refracted towardstowards NormalNormal when moving into shallower or denser when moving into shallower or denser region (frequency unchanged, speed & region (frequency unchanged, speed & wavelength decrease), and wavelength decrease), and awayaway from from NormalNormal the other way round. the other way round.

But sound waves behave But sound waves behave in the opposite way.in the opposite way.

(Try relate it to the(Try relate it to theresistancesresistances of medium) of medium)

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Diffraction of Diffraction of WavesWavesDiffractionDiffraction is the is the spreadingspreading of waves after of waves after

passing a narrow slit or an obstacle.passing a narrow slit or an obstacle.

Waves are diffracted Waves are diffracted moremore if: if:a.a. the slit is the slit is narrowernarrowerb.b. the wavelength is the wavelength is longerlonger

Light has Light has very shortvery short wavelengths, diffraction wavelengths, diffraction occurs only for slits with occurs only for slits with width of 10width of 10-4-4 m or less. m or less.

Sound has Sound has longlong wavelengths, diffraction of wavelengths, diffraction of sound waves enable sound to go around sound waves enable sound to go around cornerscorners..

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Interference of Interference of WavesWavesPrinciple of superposition of Principle of superposition of

waveswavesDisplacement y, produced by 2 waves at a Displacement y, produced by 2 waves at a point is the vector point is the vector sumsum of displacements y of displacements y11 & & yy22 produced by each of the waves. produced by each of the waves.

y = yy = y11 + y + y22

InterferenceInterference = effect produced by = effect produced by superposition of waves from 2 coherent superposition of waves from 2 coherent sources. sources.

Coherent sourcesCoherent sources = if they oscillate in = if they oscillate in phase, or antiphase, or with same phase phase, or antiphase, or with same phase difference & they have the same frequency.difference & they have the same frequency.

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Interference of Interference of WavesWavesantinodeantinode: :

constructive superposition from 2 crests or constructive superposition from 2 crests or troughstroughs

nodenode: : destructive superposition from crest & troughdestructive superposition from crest & trough

xx

aa

DD

SeparationSeparation between 2 between 2 consecutive consecutive antinodal or nodal antinodal or nodal lines at distance lines at distance DD from from the 2 sources:the 2 sources:

antinodal antinodal linelinenodal nodal

lineline

λλDDaa

x =x =Young’s double-slit Young’s double-slit

experimentexperiment

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Sound WavesSound WavesSound waves Sound waves are produced by vibrating are produced by vibrating sources. They are sources. They are longitudinallongitudinal waves waves requiring a requiring a mediummedium for propagation. It for propagation. It cannot travel in vacuum.cannot travel in vacuum.Applications Applications of reflection of sound wave:of reflection of sound wave:

a.a. sonarsonar or high frequency sound waves or high frequency sound waves used to determine used to determine depthdepth of ocean of ocean

b.b. UltrasonographyUltrasonography uses ultrasound waves uses ultrasound waves to examine condition of to examine condition of fetusfetus, the , the reflected waves used to form an image on reflected waves used to form an image on a monitor.a monitor.c.c. UltrasoundUltrasound is also used to detect is also used to detect tumourtumour in the body in the body

d.d. Reflection of ultrasound from rocks used in Reflection of ultrasound from rocks used in geological surveygeological survey..

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Electromagnetic Electromagnetic WavesWavesCommon properties:Common properties:

a.a. Need no medium, can travel in Need no medium, can travel in vacuumvacuum..

b.b. Travels at constant speed of Travels at constant speed of lightlight, , c = 3 x 10c = 3 x 1088 m s m s-1-1 in vacuum. in vacuum.

c.c. TransverseTransverse waves. waves.

d.d. Wave properties like reflection, refraction, Wave properties like reflection, refraction, diffraction & interference.diffraction & interference.

Name Sources Applications

Radio waves, microwaves Inductor-capacitor circuit Radio, radar

Infra-red Hot objects, Sun Heating, remote control, night vision

Visible light Sun, lamps Photography, fibre optics

Ultraviolet Very hot bodies, Sun, tubes Fluorescent lamp, sterilizer, sun bed

X-rays X-ray tube X-ray photography, radiotherapy, detection of cracks

Gamma rays Radioactive nuclei Radiotherapy, sterilizer

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SummarySummary

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What you have learned:What you have learned:

1.1. Understanding WavesUnderstanding Waves

2.2. Reflection of wavesReflection of waves

Thank YouThank You

3.3. Refraction of wavesRefraction of waves

4.4. Diffraction of wavesDiffraction of waves

5.5. Interference of wavesInterference of waves

6.6. Sound & electromagnetic wavesSound & electromagnetic waves