Waves!Waves!

I. A little vocab…I. A little vocab…• WAVE: WAVE:

– A periodic disturbance in a solid, liquid A periodic disturbance in a solid, liquid or gasor gas

• MEDIUM:MEDIUM:– Matter or space through which waves Matter or space through which waves

traveltravel

II. Types of wavesII. Types of waves1.1. Waves can be classified by Waves can be classified by

what they move throughwhat they move through

a.a. Mechanical- Requires a Mechanical- Requires a mediummedium

b. Electromagnetic-Doesb. Electromagnetic-Does

not require a not require a mediummedium– Ex- Ex- light waveslight waves, radio , radio

waves, ultraviolet waves, ultraviolet waves…waves…

2. Waves can also be 2. Waves can also be classified by how they moveclassified by how they move

a. Transversea. Transverse waves – travel perpendicular to waves – travel perpendicular to the direction of the wavethe direction of the wave

b. Longitudinal waves (aka b. Longitudinal waves (aka compression waves )compression waves )

travel travel parallelparallel to the direction of the to the direction of the wavewave

Surface Waves: Have BOTHSurface Waves: Have BOTH

• Occur at the border Occur at the border between two between two mediums…mediums…

•Water waves: Particles Water waves: Particles at the surface move at the surface move parallel and parallel and perpendicular (in perpendicular (in circles)circles)

•The ball up and down The ball up and down and side to sideand side to side

Draw your picturesDraw your pictures

Waves Carry EnergyWaves Carry Energy• Simple Harmonic MotionSimple Harmonic Motion- When you pull on a single spring, - When you pull on a single spring,

its mass continues to bounce up and down.its mass continues to bounce up and down.

• When pulled down, spring gains PE (elastic) As it moves back up when When pulled down, spring gains PE (elastic) As it moves back up when released, PE is transferred to KEreleased, PE is transferred to KE

• Mass goes past the original starting positionMass goes past the original starting position

• Back at the top, all energy is transferred to PE (gravitational & elastic Back at the top, all energy is transferred to PE (gravitational & elastic PE)PE)

• When it moves down again, energy transfers back to KE and it keeps When it moves down again, energy transfers back to KE and it keeps goinggoing

Damped Harmonic Motion-Damped Harmonic Motion- a a vibration that vibration that fades outfades out as energy as energy is transferred from one object to is transferred from one object to another another • When pulled down, the When pulled down, the

spring gains PEspring gains PE

• Again, it will be Again, it will be converted to KE when converted to KE when releasedreleased

• BUT instead of that KE BUT instead of that KE staying in that spring, it staying in that spring, it is transferred to the is transferred to the next spring in the line.next spring in the line.

Anatomy of a Transverse Anatomy of a Transverse WaveWave

Crest:Crest:Trough: Trough:

Anatomy of a Longitudinal Anatomy of a Longitudinal WaveWave

Compression:Compression:Rarefaction: Rarefaction:

Inquiry LessonInquiry Lesson

How do we describe waves?How do we describe waves?

• WavelengthWavelength

• AmplitudeAmplitude

• PeriodPeriod

• FrequencyFrequency

WavelengthWavelength ( ())

– The distance between two The distance between two identicalidentical parts parts of a waveof a wave

• The distance btwn 2 crests for a The distance btwn 2 crests for a TRANSVERSE wave or two compressions for a TRANSVERSE wave or two compressions for a LONGITUDINAL wave. LONGITUDINAL wave.

– The symbol for wavelenths is ____The symbol for wavelenths is ____

Draw 2 waves – one with a Draw 2 waves – one with a long wavelength and one with long wavelength and one with a short wavelength!a short wavelength!

AmplitudeAmplitude– Distance from the middle of the wave to the Distance from the middle of the wave to the

crest crest • (or half the distance between the crest and the trough)(or half the distance between the crest and the trough)

– Larger waves have larger Larger waves have larger amplitudesamplitudes and and carry more energy carry more energy

– Sound Energy: The more amps the sound Sound Energy: The more amps the sound has, the ________________ it will be.has, the ________________ it will be.

– (usually represented by the line denisty for a (usually represented by the line denisty for a compressional wave)compressional wave)

Low amplitudeLow amplitude Medium Amplitude Medium Amplitude HIGH HIGH AmplitudeAmplitude

Draw 2 waves – one with an Draw 2 waves – one with an amplitude of 5 mm and one amplitude of 5 mm and one with an amplitude of 2 mmwith an amplitude of 2 mm

PeriodPeriod

• The time required for 1 full The time required for 1 full wavelengthwavelength of a wave to pass a of a wave to pass a certain pointcertain point

– Or the time required to complete one Or the time required to complete one vibrationvibration of a particle in a medium of a particle in a medium

• Represented by “t” (Represented by “t” (secondsseconds) )

FrequencyFrequency

The number of The number of cyclescycles/wavelengths/vibrations /wavelengths/vibrations per unit of timeper unit of time

– Represented by “f” ; f= 1/tRepresented by “f” ; f= 1/t– Measured in Measured in HertzHertz (named after Heinrich Hertz – 1888 – 1st (named after Heinrich Hertz – 1888 – 1st

demonstrated the presence of electromagnetic waves)demonstrated the presence of electromagnetic waves)– Equal to vibrations per Equal to vibrations per secondsecond (1Hz = 1 vibration/1 second) (1Hz = 1 vibration/1 second)

• You can hear 20 Hz to 20,000 HzYou can hear 20 Hz to 20,000 Hz

• More More vibrationsvibrations = = shortershorter amount of time for each amount of time for each

1. Draw a wave with a period 1. Draw a wave with a period of .5 seconds and a frequency of .5 seconds and a frequency of 2 Hzof 2 Hz

2. Draw 2 waves – one with a 2. Draw 2 waves – one with a high frequency and one with a high frequency and one with a low frequency:low frequency:

Frequency of Light Spectrum Frequency of Light Spectrum (example)(example)

• Wide range of frequencies and wavelengthsWide range of frequencies and wavelengths

• Your eyes can detect 4.3 x 1014 Hz to 7.5 x 1014 Hz Your eyes can detect 4.3 x 1014 Hz to 7.5 x 1014 Hz (visible light range)(visible light range)

• Electromagnetic waves exist at frequencies we can’t seeElectromagnetic waves exist at frequencies we can’t see

Remember that each wave Remember that each wave carries energy…carries energy…

• Higher frequency = Higher frequency = moremore energy and energy and shortershorter wavelength…The more wavelength…The more wavelengths per unit time, the more wavelengths per unit time, the more energy.energy.

Ex- sound waves…Ex- sound waves…•Sound waves – travel as spherical Sound waves – travel as spherical

waves from a speakerwaves from a speaker

•Wave front – each circular waveWave front – each circular wave

•As they travel out, the wave front As they travel out, the wave front gets bigger – each front has the gets bigger – each front has the same amount of energy. As the same amount of energy. As the wave front gets bigger, the energy wave front gets bigger, the energy is spread out over a bigger area is spread out over a bigger area (so that is why these waves are (so that is why these waves are damaging close-up –close to damaging close-up –close to speaker or close to light)speaker or close to light)

Wave Wave speedspeed – how fast a wave – how fast a wave movesmoves

• Speed = Speed = wavelengthwavelength / / periodperiod

• So…So…

• Wave speed = Wave speed = frequencyfrequency x x wavelengthwavelength (because the period is (because the period is the inverse of the frequencythe inverse of the frequency

Wave speed depends on Wave speed depends on mediummedium SoundSound waves can travel in air waves can travel in air

However, they travel even faster in However, they travel even faster in solidssolids than in air or water than in air or water

Example: hitting a metal railing and Example: hitting a metal railing and listening (you will hear two bangs)listening (you will hear two bangs)

Why?Why?

The first sound comes through the steel The first sound comes through the steel rail itself and reaches you shortly rail itself and reaches you shortly before the second sound, which before the second sound, which travels through the air.travels through the air.

Wave speed depends on Wave speed depends on mediummedium

In a gas, particles are farther apart so it In a gas, particles are farther apart so it takes longer for them to find each other takes longer for them to find each other and bump into each other - sending the and bump into each other - sending the wavewave

• In liquids, particles are In liquids, particles are closercloser together so together so waves travel faster waves travel faster

• In solids, particles are very In solids, particles are very closeclose together together so waves travel the fastest so waves travel the fastest

Example: EarthquakesExample: Earthquakes(you don’t need to know (you don’t need to know

this…yet)this…yet)

Produce Produce seismicseismic waves waves

P waves (P waves (primaryprimary) – travel faster, arrive ) – travel faster, arrive first, longitudinal waves (shake first, longitudinal waves (shake everything side-to-side)everything side-to-side)

S waves (S waves (secondarysecondary) – slower, carry more ) – slower, carry more energy, transverse waves (shake energy, transverse waves (shake everything up and down – more everything up and down – more damage) damage)

10. Doppler Effect10. Doppler Effect

– First – pitch is determined by First – pitch is determined by frequencyfrequency– Sound waves travel out from an object in wave fronts Sound waves travel out from an object in wave fronts

and hit your eardrum (when they hit your eardrum, they and hit your eardrum (when they hit your eardrum, they have a frequency equal to the number of wave fronts have a frequency equal to the number of wave fronts that hit your earthat hit your ear

– Ambulance example: Ambulance example: • When the ambulance moves toward you, the sound waves When the ambulance moves toward you, the sound waves

are compressed because the ambulance moves a short are compressed because the ambulance moves a short distance – so the waves hit your eardrum at a higher distance – so the waves hit your eardrum at a higher frequency – higher pitchfrequency – higher pitch

• Then, when the ambulance moves away from you, the Then, when the ambulance moves away from you, the distance between the wave fronts is more than before, so distance between the wave fronts is more than before, so the frequency is lower – pitch is lowerthe frequency is lower – pitch is lower

– Doppler effect can also happen in light and other types Doppler effect can also happen in light and other types of wavesof waves

• http://physics.bu.edu/~duffy/semester1/c20_wave_speed.html

• wave speed different mediumwave speed different medium• http://www.teachersdomain.org/resource/http://www.teachersdomain.org/resource/

ess05.sci.ess.watcyc.wavemotion/ess05.sci.ess.watcyc.wavemotion/• surface wavesurface wave• http://www.acoustics.salford.ac.uk/feschools/http://www.acoustics.salford.ac.uk/feschools/

waves/wavetypes.htmwaves/wavetypes.htm• transverse right anglestransverse right angles• http://www.acoustics.salford.ac.uk/feschools/http://www.acoustics.salford.ac.uk/feschools/

waves/reflect.htm#reflectwaves/reflect.htm#reflect• reflection reflection