Waves

WavesWaves

Unit 8

Chapter 11

Topics to be covered in this unitTopics to be covered in this unit

1. Types of Waves 1. Types of Waves

2. Characteristics of Waves 2. Characteristics of Waves

3. Wave Interactions 3. Wave Interactions

What is a wave?What is a wave?

• Waves are disturbances that transmit energy through matter or space.– Example – when you throw a stone into a

pond… it creates a ripple on the surface of the water.

How waves travelHow waves travel

• Ripples in the pond

• Sound traveling through air

• Earthquakes in the ground

• These are all examples of mediums in which waves can travel.

• A medium is the matter through which a wave travels.

How waves travelHow waves travel

• Waves that require a medium are called mechanical waves.

• Almost all waves require a medium– Exceptions are electromagnetic waves

Electromagnetic waveElectromagnetic wave

• Electromagnetic wave is a wave caused by a disturbance in electric and magnetic fields and does not require a medium.

LightLight

• Light can travel from the sun to the Earth through the empty space between the two.

• Light does not need a medium

• Light waves consist of changing electric and magnetic fields in space.

• They are also called electromagnetic waves– Examples: visible light and radio waves

Waves transfer energyWaves transfer energy

• Remember – energy is the ability to do work.

• Waves carry energy because they can do work.– Water waves can move a boat or leaf– Sound waves impact your eardrum– Light waves work on your eye

• The bigger the wave, the more energy it carries.

TsunamiTsunami

Waves transfer energyWaves transfer energy

• Tsumani waves carry enough energy to cause a lot of damage to coastal towns and shorelines.

• Normal-size waves do work on the shore to break up rocks into tiny pieces forming sandy beaches.

Energy may spread out as a wave travelsEnergy may spread out as a wave travels

• The closer you are to speakers, the louder the music.

• Waves spread out in circles that get bigger as the waves move farther from the center.

• Each circle (called a wave front) has the same amount of total energy. But as the circle get larger, the energy spreads out over a larger area.

Vibrations and WavesVibrations and Waves

• Waves are related to vibrations– Example: a singer sings (vocal cords move back and

forth) creating sound. The sound waves reach your ears causing your eardrum to vibrate. This vibration is interpreted by your brain as sounds.

• Most waves are caused by a vibrating object• Electromagnetic waves may be caused by

vibrating charged particles

• In a mechanical wave, the particles in the medium also vibrate as the wave passes through the medium.

Vibrations involve transformations of energy


• Vibration: in a general sense, anything that switches back and forth, to and from, side to side, in and out, off and on, loud and soft, or up and down is vibrating. A vibration is a wiggle in time.

• Wave: a wiggle in both space and time is a wave. A wave extends from one place to another.



• Vibrations and waves: the source of all waves is something that is vibrating. Waves are propagations of vibrations throughout space.

• Example is a pendulum in a clock.

simple harmonic motion is a type of motion where the restoring force is

directly proportional to the displacement

simple harmonic motion is a type of motion where the restoring force is

directly proportional to the displacement

Simple Harmonic MotionSimple Harmonic Motion

• A wave whose source vibrates with simple harmonic motion is called a sine wave

• As the mass moves down, the spring is exerting force which pushes it back to the original position. This will allow the mass to keep bouncing up and down forever.

Damped Harmonic MotionDamped Harmonic Motion

• Unlike the simple harmonic motion which will continue forever. The damped harmonic motion will transfer its energy to the second mass, it slows down and then returns to its resting position.

• http://www.animations.physics.unsw.edu.au/jw/oscillations.htm#Damped

Wave TypesWave Types

Transverse waves vibrate across from direction of travel

Longitudinal waves vibrate along the direction of travel (as in a spring)

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Longitudinal WaveLongitudinal Wave

• The wave we see here is a longitudinal wave.

• In a longitudinal wave, the medium particles vibrate parallel to the motion of the pulse.

• This is the same type of wave that we use to transfer sound.

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Transverse wavesTransverse waves

• A second type of wave is a transverse wave.

• We said in a longitudinal wave the pulse travels in a direction parallel to the disturbance.

• In a transverse wave the pulse travels perpendicular to the disturbance.

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What’s the difference?What’s the difference?

• The differences between the two can be seen

Surface WaveSurface Wave

• Surface waves occur at the boundary between two different mediums such as between water and air.

• Example: The waves on the ocean are not simply transverse or longitudinal waves.

Characteristics of WavesCharacteristics of Waves

• Waves can be observed with many different properties:– Large or small– Close together or very far apart

• However, they can all be described with their properties using a sine curve.

Wave PropertiesWavelength

Wave PropertiesWavelength

Wavelength,, is the distance between any two successive identifical parts of a wave.

Wave PropertiesAmplitude

Wave PropertiesAmplitude

Amplitude is the greatest distance that particles in a medium move from their normal position when a wave passes.

Parts of a WaveParts of a Wave

• Crest: The highest point of a transverse wave

Parts of a WaveParts of a Wave

• Trough: The lowest point of a transverse wave.

• These are the points where the wave exhibits its maximum negative or downward displacement.

Longitudinal wave parts:Longitudinal wave parts:

• Compressions- a region in a longitudinal wave where the particles are closest together. It is an area of high density

• Rarefactions- A decrease in density and pressure in a medium.

Wave PropertiesPeriod

Wave PropertiesPeriod

The wave period, P, the time required for one full wavelength to pass a certain point.

Wave PropertiesFrequency

Wave PropertiesFrequency

Frequency, f, is the number of vibrations that occur in a 1 second time interval.

Frequency-Period EquationFrequency-Period Equation

• The frequency and period of a wave are related. – If more vibrations are made in a second, each

one takes a shorter amount of time. – The frequency is inverse of the period.

• SI unit is Hertz (Hz)

Frequency = 1

period

f = 1/T

Visible LightVisible Light

• Our eyes can detect light with frequencies from 4.3 x 1014 Hz to 7.5 x 1014 Hz.

• This is called the visible light

Electromagnetic SpectrumElectromagnetic Spectrum

• Light occurs at other ranges we cannot see with our eyes.

• The full range is called the electromagnetic spectrum.

Type of wave

Mechanical Electromagnetic

Form Longitudinal Transverse Modeled as transverse

Medium Solids, liquids, gases

Solids and liquids

None required

Travel as Compression and rarefactions in matter

Back & forth movement of matter

Oscillating electric and magnetic fields

Examples Sound waves and some earthquake waves

Water waves, rope waves, some earthquake waves

Visible light waves, radio waves, X-rays

Wave speedWave speed

• Wave speed is the speed at which a wave passes through a medium.

• Formula:

Wave speed = frequency x wavelength

v = f x

SI unit for wave speed is meters per second.

m/s

Practice problemPractice problem

• The average wavelength in a series of ocean waves is 15.0 m. A wave arrives on average every 10.0 s, so the frequency is 0.100 Hz. What is the average speed of the waves?

• v = f x

Practice problemPractice problem

• The average wavelength in a series of ocean waves is 15.0 m. A wave arrives on average every 10.0 s, so the frequency is 0.100 Hz. What is the average speed of the waves?

v = f x = 0.100 Hz x 15.0 m

= 1.5 m/s

Practice problemsPractice problems

v = f x

P = 1

t

Sound WavesSound Waves

Molecules in the air vibrate about some average position creating the compressions and rarefactions. We call the frequency of sound the pitch.

Doppler EffectDoppler Effect

• Doppler effect is an observed change in the frequency of a wave when the source or observer is moving.

• Pitch of a sound, how high or low it is, is determined by the frequency at which sound waves strike the eardrum in your ear.

Of course…..Of course…..

• There's this take on it

Doppler Effect Doppler Effect

• Refers to the change in frequency when there is relative motion between an observer of waves and the source of the waves

• Doppler with Sound

Red shift?Red shift?

Expand that…..Expand that…..

• Wrap your mind around this

Wave InteractionsWave Interactions

• Two different material objects can never occupy the same space at the same time.

• When two waves come together they do not bounce back as bumper boats.

• Because mechanical waves are not matter but rather the displacement of matter, two waves can occupy the same space at the same time


• Reflection is the bouncing back of a wave as it meets a surface or boundary

angle of incidence=angle of reflection

Reflection of a wave from a free end

Reflection of a wave from a free end

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com

Reflection of a wave from a fixed end

Reflection of a wave from a fixed end

Animations courtesy of Paul Hewitt and borrowed from physicsclassroom.com


• Diffraction is the bending of a wave as it passes an edge or an opening


• Refraction is the bending of waves as the pass from one medium to another.

What is the difference?What is the difference?

• http://www.brainpop.com/science/energy/refractionanddiffraction/

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Wave InteractionWave Interaction

• All we have left to discover is how waves interact with each other.

• When two waves meet while traveling along the same medium it is called INTERFERENCE.

• Interference is the combination of two or more waves that exist in the same place at the same time.

Wave InterferenceWave Interference

When two wave pass each other their superposition causes reinforcement or cancellation.

Constructive interferenceConstructive interferenceReinforcement when the crest of one wave

overlaps the crest of another Their individual effects adds together, resulting

in a wave increased in amplitudeIn phase- two waves crests and troughs arrive at a place at the same time, effects reinforce each other

Destructive InterferenceDestructive InterferenceCancellation when crest of one wave overlaps trough

of another reducing their individual effectsWater waves show these best

Out of phase- the crest of one wave arrives at a point at the same time as a trough of the second wave arrives, effects cancel each other

Sound Wave InterferenceSound Wave Interference

•Interference occurs when two sounds of difference frequency are heard superposed. •Constructive interference (compressions align) causes louder sound and destructive inference cause fainter sound•This alternating pattern produces a beat.

Shock WavesShock Waves

Water Wave InterferenceWater Wave Interference

• Left side is theoretical drawing of an interference pattern.

• Right side is the actual interference pattern.

Standing WavesStanding WavesStanding wave is a wave form caused by

interference that appears not to move along the medium and that shows some regions of no vibrations (nodes)….

Standing WavesStanding WavesAnd other regions of maximum vibrations called antinodes.Antinodes form where the crests of the original waves line

up with the crests of the reflected waves so that complete constructive interference occurs.

Standing WavesStanding Waves

Change the frequency in a standing wave and more nodes/antinodes appear in the event

Review QuestionsReview Questions

What dictates the frequency of a sound wave?

What dictates the frequency of a sound wave?

(a) wavelength

(b) medium

(c) source of the sound

(d) speed

(e) amplitude

What determines the speed of a wave?

What determines the speed of a wave?

(a) the frequency

(b) the wavelength

(c) the amplitude

(d) the period

(e) the medium of transmission

A skipper on a boat notices wave crests passing his anchor chain

every 5 seconds. If the wave crests are 15 m apart, what is the speed

of the water waves in m/s?

A skipper on a boat notices wave crests passing his anchor chain

every 5 seconds. If the wave crests are 15 m apart, what is the speed

of the water waves in m/s?

(a) 5 (d) 10

(b) 15 (e) 3

(c) 75

For a medium transmitting a longitudinal wave, the areas of the medium where the density of the medium is temporarily increased

are called...

For a medium transmitting a longitudinal wave, the areas of the medium where the density of the medium is temporarily increased

are called...

(a) rarefactions

(b) compressions

(c) density holes

When you move away from a fixed source of sound, the frequency of

the sound you hear...

When you move away from a fixed source of sound, the frequency of

the sound you hear...

(a) is greater than what the source emits

(b) is less than what the source emits

(c) is the same as what the source emits

Sonic booms from a plane are produced...

Sonic booms from a plane are produced...

(a) because the plane breaks through the sound

barrier.

(b) when the plane reaches the speed of sound.

(c) by the plane traveling faster than the speed of

sound.

(d) by the plane traveling slower than the speed

of sound.

Type of wave

Mechanical Electromagnetic

Form Longitudinal Transverse Modeled as transverse

Medium Solids, liquids, gases

Solids and liquids

None required

Travel as Compression and rarefactions in matter

Back & forth movement of matter

Oscillating electric and magnetic fields

Examples Sound waves and some earthquake waves

Water waves, rope waves, some earthquake waves

Visible light waves, radio waves, X-rays

Waves

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Transcript of Waves