Chapter 17 Mechanical Waves and Sound Mechanical Waves Section 17-1.
Waves Classification of Waves Waves can be classified as either mechanical or electromagnetic....
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Transcript of Waves Classification of Waves Waves can be classified as either mechanical or electromagnetic....
Waves
Classification of Waves• Waves can be classified as either
mechanical or electromagnetic.• Mechanical wave examples: water waves,
waves on a spring, sound waves and ultrasonic waves.
• Mechanical waves must have a medium to travel through and cannot travel in a vacuum.
• A mechanical wave passing through a medium is vibrations being passed on from molecule to molecule.
Classification of Waves
• Electromagnetic wave examples: radio waves, microwaves, infra-red waves, ‘visible’ light waves, ultraviolet waves, X-rays and gamma-rays.
• Electromagnetic waves can travel through a vacuum and do not need a medium to travel through.
• Electromagnetic waves travel fastest in a vacuum at a speed of 3 x 108 metres per second (speed of light).
Waves on a Spring• If you hold a number of coils together – called
a compression – let them go and the compression moves along the spring.
• After the compression passes a point on the spring, the coils in that part become stretched more than normal.
• This is called a rarefaction.
Waves Are a Means of Transferring Energy• When waves move along water or a rope, there
is no overall motion as the wave passes.• As the wave pulse passes a point, the medium
around the point is disturbed, but when the pulse has passed, the medium at that point is no longer moving.
• A Travelling Mechanical Wave is a disturbance carrying energy through a medium without any overall motion of that medium.
Electromagnetic Waves• When an electromagnetic wave passes
through a region of space, there is a rapidly changing electric and magnetic field in that region.
• By this means, energy gets transferred from one place to another by the wave (Heat energy).
• A travelling wave, either mechanical or electromagnetic, is a disturbance that travels out from the source producing it, transferring energy from the source to other places through which it passes.
Some definitions…
1) Amplitude – this is height of the wave.
2) Wavelength () – this is the distance between two corresponding points on the wave and is measured in metres:
3) Frequency – this is how many waves pass by a point every second and is measured in Hertz (Hz)
Crest
Trough
Longitudinal Wave
Some definitions…Transverse waves are when the displacement is at right angles to the direction of the wave…
Longitudinal waves are when the displacement is parallel to the direction of the wave…
e.g.Lighte.g.Light
e.g.Soune.g.Soundd
Transverse waves are when the oscillation is at 90o to the direction of propagation
Longitudinal waves are when the oscillation is parallel to the direction of propagation
“Seeing” a wave
1) Quiet sound, low frequency (i.e. high wavelength):
2) Quiet sound, high frequency (i.e. low wavelength):
3) Loud sound, low frequency:
4) Loud sound, high frequency:
The Wave EquationThe wave equation relates the speed of the wave to its frequency and wavelength:
Wave speed (v) = frequency (f) x wavelength ()
in m/s in Hz in m
V
f
f
Using this formula we can convert any wavelength to
a frequency.
Remember Frequency – this is how many waves pass by a point every second and is measured in Hertz (Hz)
1) A water wave has a frequency of 2Hz and a wavelength of 0.3m. How fast is it moving?
2) A water wave travels through a pond with a speed of 1m/s and a frequency of 5Hz. What is the wavelength of the waves?
3) The speed of sound is 330m/s (in air). When Dave hears this sound his ear vibrates 660 times a second. What was the wavelength of the sound?
4) Purple light has a wavelength of around 6x10-7m and a frequency of 5x1014Hz. What is the speed of purple light?
Some example wave equation questions
0.2m
0.5m
0.6m/s
3x108m/s
Reflection• Reflection is when a wave meets an
obstacle in its path, it bounces off that obstacle.
• This can be seen with a water wave, the reflection of light waves in a mirror and sound waves through an echo.
Reflection
Waves Changing Speed• Waves change speed when they go
from one medium to another.
• Their frequency remains the same.• The wavelength increases if the wave
speeds up and the wavelength decreases if the wave slows down.
Refraction through a glass Refraction through a glass block:block:
Wave slows down and bends towards the normal due to
entering a more dense medium
Wave speeds up and bends away from the normal due to
entering a less dense medium
Wave slows down but is not bent, due to
entering along the normal
RefractionRefraction is when waves ____ __ or slow down due to travelling in a different _________. A medium is something that waves will travel through.
In this case the light rays are slowed down by the water and are _____, causing the ruler to look odd. The two mediums in this example are ______ and _______.
Words – speed up, water, air, bent, medium
The wavelength also changes.
Wave diagrams1) Reflection
4) Diffraction3) Refraction
2) Refraction
Diffraction
More diffraction if the size of the gap is similar to the wavelength
More diffraction if wavelength is increased (or frequency decreased)
Diffraction is when waves spread out from the edge of a gap.
Sound bends better around corners
Interference of Waves• Interference is when two waves from
two sources meet and a new wave is produced.
• When waves arrive crest with crest and trough with trough, they are said to be in phase.
Interference of Waves
Finding the Critical Angle…
1) Ray gets refracted
4) Ray gets internally reflected3) Ray still gets refracted (just!)
2) Ray still gets refracted
THE CRITICAL ANGLE
Uses of Total Internal ReflectionOptical fibres:
An optical fibre is a long, thin, transparent rod made of glass or plastic. Light is internally reflected from one end to the other, making it possible to send large chunks of information
Optical fibres can be used for communications by sending e-m signals through the cable. The main advantage of this is a reduced signal loss. Also no magnetic interference.
It is important to coat the strand in a material of low n.
The light can not leak into the next strand.
Other uses of total internal reflection1) Endoscopes (a medical device used to see inside the body):
2) Binoculars and periscopes (using “reflecting prisms”)
How does ultrasound work?
Ultrasonic waves are partly _________ at the boundary as they pass from one _______ to another. The time taken for these reflections can be used to measure the _______ of the reflecting surface and this information is used to build up a __________ of the object.
Words – depth, reflected, picture, medium
Ultrasound is the region of sound above 20,000Hz – it can’t be heard by humans. It can be used in pre-natal scanning:How does it work?
Other uses of ultrasound1) Echo sounding
The ultrasound is reflected from the sea floor.
2) Breaking down kidney stonesUltrasonic waves break kidney stones into much smaller pieces
3) Cleaning (including teeth)Ultrasound causes dirt to vibrate dirt off without damaging the object
The electromagnetic spectrum
Gamma rays
X-rays Ultra violet Visible light
Infra red Microwaves
Radio/TV
Each type of radiation shown in the electromagnetic spectrum has a different wavelength and a different frequency:
Each of these types travels at the same speed through a vacuum and can be polarised. Different wavelengths are absorbed by different surfaces (e.g. infra red is absorbed very well by black surfaces). This absorption may heat the material up (like infra red and microwaves) or cause an alternating current (like in a TV Ariel).
High frequency, short wavelength
Low frequency,long wavelength
γ
The higher the frequency of the wave, the greater its energy. This makes X-rays dangerous and radio waves
safe
Detection
• Waves invisible to the eye have to be detected using special apparatus
• IR (Infra-Red) is a heat wave so a blackened thermometer bulb
Night Vision Camera
• Of course we could just skip forward 100years
UV Light
• Ever walked into a nightclub• White cloth washed in optical
brighteners glows in UV light
Gamma• Bubble chambers where the wave
leaves a trail of bubbles
How Microwaves and Infra-red workMicrowaves are absorbed by water molecules up to a depth of a few centimetres. The heat then reaches the centre of the food by conduction.
Infra-red waves are absorbed by the surface of the material and the energy is then passed to the centre of the food by conduction.
The higher the frequency of the wave, the greater its
energy
X-rays and gamma () raysX-rays are absorbed by ____ parts of the body, like ____. Unfortunately, over-exposure to x-rays will damage cells.
Gamma rays can be used to treat _______. A gamma ray source is placed outside the body and rotated around the outside of the tumour. Doing this can ___ the cancerous cells without the need for ______ but it may damage other cells and cause sickness.
Tracers can also be used – these are small amounts of ___________ material that can be put into a body to see how well an organ or ______ is working.
Words – radioactive, gland, cancer, hard, bones, kill, surgery
Sun is not Yellow
As the light is filtered through As the light is filtered through more atmosphere more more atmosphere more frequencies absorbedfrequencies absorbed
Sky appears blue as scattered blue light from sun appears Sky appears blue as scattered blue light from sun appears to be coming from lots of different directionsto be coming from lots of different directions
Wave 2
Resultant wave
Wave 1
Coherent Waves
• Same Frequency• In Phase
Or Constant Or Constant phase differencephase difference
Phase difference Phase difference in measured in in measured in degrees of a degrees of a circlecircle
Coherent Waves
• Same Frequency• In Phase
Or Constant Or Constant phase differencephase difference
Phase difference Phase difference in measured in in measured in degrees of a degrees of a circlecircle
Interference is where 2 coherent waves meet. The resultant is the algebraic sum of
the 2 waves at any point.
+ =
Constructive Constructive InterferenceInterference
If 180 degrees out of phase.
+ =Destructive Interference
To Remember this we simplify it a To Remember this we simplify it a littlelittle
White Light Interference
To get two
coherentcoherent sources (same frequency and phase) we use one source and two slits.
ConstructiveConstructive InterferenceInterference
n=0
n=1
n=1
Proving the wave nature of Light
The interference The interference patterns prove patterns prove light is a wave.light is a wave.
Internet Example
Equation
• d = 1/(N x1000) (Grating Const lines/mm)
n=1n=1
So one wavelength So one wavelength difference difference Constructive Constructive InterferenceInterference
dd
Equation
dd
• When more than one wavelength difference
• d sin = n
• sin = /d• d sin =
For the n=2 dot
dd
22
• When n wavelength differences
• d sin = n
• sin = 2/d• d sin = 2
• What we actually see on the screen is a series of bright lines called fringes where there is constructive interference. This an interference pattern
n=0n=0
n=1n=1n=1n=1
n=2n=2n=2n=2
n=3n=3 n=3n=3
3 3 wavelengths wavelengths difference difference
in pathin path
n = 2
n = 1
n = 2
n = 1
n = 0
x
D
Laser
Metre stick
Diffractiongrating
θ
Tan θ = x/D
MEASUREMENT OF THE WAVELENGTH OF MONOCHROMATIC LIGHT
1. Set up the apparatus as shown. Observe the interference pattern on the metre stick – a series of bright spots.2. Calculate the mean distance x between the centre (n=1) bright spot and the first (n =1) bright spot on both sides of centre.3. Measure the distance D from the grating to the metre stick.4. Calculate θ.5. Calculate the distance d between the slits, using d=1/N the grating number.Calculate the wavelength λ using nλ = dsinθ.
6. Repeat this procedure for different values of n and get the average value for λ
As As nλ = dsinθ if d gets larger if d gets larger
then then θθ gets smaller gets smaller
H/W
• 2005 HL Q7
Polarization of LightNormally all e-m waves (Transverse)
oscillate in all perpendicular planes at once.
Polarization leaves only one plane of oscillation
Sound is a longitudinal wave and so can Sound is a longitudinal wave and so can not be Polarisednot be Polarised
Polarizing Filters
Hydrocarbons that absorb light that is in it’s plane of orientation.
Polarisation is the taking a transverse wave that oscillates in all perpendicular planes and filtering it so it oscillates in only one perpendicular plane.
Standing WavesWhen two coherent waves of the same amplitude traveling in opposite directions meet the waves combine to form a stationary wave
We draw this as the two extremes
nnAA
http://www.absorblearning.com/media/http://www.absorblearning.com/media/attachment.action?quick=8u&att=628attachment.action?quick=8u&att=628
Real Standing Waves
Strings
Closed Tubes
Open Tubes
/2
/2
/4
MEASUREMENT OF THE SPEED MEASUREMENT OF THE SPEED OF SOUND IN AIROF SOUND IN AIR
N
Tuning fork
Tube
Water
l1Graduated cylinder
A
MEASUREMENT OF THE SPEED OF SOUND IN AIR
Tuning forkTuning fork
TubeTube
WaterWater
ll11 Graduated Graduated cylindercylinder
dd
λλ = 4( = 4(ll11 + 0.3 + 0.3dd))
Method1. Strike the highest frequency (512 Hz)
tuning fork and hold it in a horizontal position just above the mouth of the tube.
2. Slide the tube slowly up/down until the note heard from the tube is at its loudest; resonance is now occurring.
3. Measure the length of the air column (from the water level to the top of the tube) l1 with a metre stick.
• An end correction factor has to be added to the length e = 0.3d, where d is the average internal diameter of the tube (measured using a vernier callipers).
• Hence λλ = 4( = 4(ll11 + 0.3 + 0.3dd)) • c c = = ff• c c = 4= 4ff((ll11 + 0.3 + 0.3dd).). • Calculate a value of c for each tuning fork and
find an average value for the speed of sound.
Harmonics
Whole number multiples of the fundamental frequency that happen at the same time as the fundamental.
Violin Harmonics
Viola Harmonics
You can hear You can hear the the difference as difference as the two the two instruments instruments have have different different combinations combinations of harmonicsof harmonics
Stretched String
A low note on a Double A low note on a Double Bass contains all the Bass contains all the harmonics above it.harmonics above it.
This is what gives the This is what gives the instrument its pleasant instrument its pleasant timbre or quality.timbre or quality.
Formula for stretched string
L=lengthT=tension=mass/unit length
Tf
lf
1
T
lfrequency
2
1
INVESTIGATION OF THE VARIATION OF FUNDAMENTAL FREQUENCY OF A STRETCHED
STRING WITH LENGTH
Bridge
l
Paper rider
Sonometer
Tuning Fork
Place the bridges as far apart as possible.Strike the turning fork putting the end on the bridge and reduce the length until the maximum vibration is reached (the light paper rider should jump off the wire).Measure the length with a metre rule.Note the value of this frequency on the tuning fork.Repeat this procedure for different tuning forks and measure the corresponding lengths.
Plot a graph of frequency f
against inverse of lengthl
1
l
1
f
INVESTIGATION OF THE VARIATION OF THE FUNDAMENTAL FREQUENCY OF A STRETCHED STRING WITH TENSION
Bridge
l
Paper rider
Sonometer
Pulley
Weight
•Select a wire length l (e.g. 30 cm), by suitable placement of the bridges. Keep this length fixed throughout the experiment. •Strike the tuning fork and hold it on the bridge.•Increase the tension by adding weight slowly from lowest possible until resonance occurs. (Jumping paper)•Note tension from weight used (In Newtons) and frequency from the tuning fork.
Plot a graph of frequency f
against square root of the tension
f
T
Musical Notes
Music waves have a regular Music waves have a regular shape where noise is shape where noise is
irregularirregularThree Qualities – called the characteristics
1. Pitch - This is frequencyfrequency of the wave.
2. Loudness - this is the amplitudeamplitude of the wave.
3. Timbre or Quality - The wave shape that is mainly due its overtonesovertones.
Demo
• Oscilloscope and microphone
Resonance• Transfer of energy between two
objects with the same, or very similar, natural frequency.
Barton’s Pendulum
String
Resonance
• If we set the driver in motion
Resonance• The energy is transferred only to
the pendulum of the same length.
Barton’s Pendulum
Resonance
• And back again for a remarkably long time.
A Stationary Source
• The waves radiate out from the source
• The wavelength detected at A is the same as at B
A moving Source
• The waves still radiate out from the source
• The wavelength detected at A is the longer than that at B
Movement Movement of sourceof source
Doppler Effect
The apparent change in frequency due to the motion of the observer or the source• Hence the change in pitch as a car passes
• Used by the Gardai in to detect speeding cars
Red Shift of Stars (Doppler in Light)
The Sun
Oh Bugger!
Moved to longer wavelengths proving the star is moving away from us
Example.A train emits a whistle at 700Hz what is the apparent frequency if it is traveling towards you at 30m/s? (c=340m/s)
Using f’ = f.c/(c-v)
f’ = 700.340/(340-30)
= 767 HzWhere f= Source Frequency and f’=Apparent FrequencyWhere f= Source Frequency and f’=Apparent Frequency
C=Speed of Wave and v=Speed of ObjectC=Speed of Wave and v=Speed of Object
H/W
• 2003 HL Q7
Tuning Forks - Both prongs vibrate and create sound
Summary - Sound as a WaveInterference proves sound is a wave.
If we twist a tuning fork near our ear it goes loud and soft.
The two prongs of the fork are interfering with each other.
LLOOUUDD
SSOOFFTT
LLOOUUDD
LLOOUUDD
LLOOUUDD
SSOOFFTT
SSOOFFTT
SSOOFFTT
Sound Intensity Level• This is to measure the very large range of
energy levels the ear can respond to, measured in decibels (dB). This is an exponential scale so if the energy doubles the level goes up by e dB.
• Home CD player 75 dB tops but a good rock band maybe 110dB.
• Health and safety tell us that if you stay in an environment above 85dB for more than 8 hrs you do permanent and un-repairable damage to your ears. So Muse is right out.
Sound Intensity level
• Also called acoustic intensity level is a logarithmic measure of the sound intensity in comparison to the reference level of 0 dB (decibels).
• The measure of a ratio of two sound intensities is
• where J1 and J0 are the intensities.
• The sound intensity level is given the letter "L J" and is measured in "dB". Decibels (dB) are dimensionless.
• If J0 is the standard reference sound intensity, where
• (W = watt), then instead of "dB" we use "dB SIL". (SIL = sound intensity level).
• We also have dBA, which is adjusted to allow for the range of the human ear.
Acoustics• Use reflections and direct sound to
amplify sound in a concert hall.
To achieve a loud sound: * If necessary, reflectors and diffusers
may be used to provide beneficial supporting sound reflections
* The interior surfaces of the hall should be hard to ensure that sound energy is not absorbed and lost.
Threshold of Hearing
• The absolute threshold of hearing (ATH) is the minimum sound level of a pure tone that an average ear with normal hearing can hear in a noiseless environment at 1kHz.
Limit of Audibility• The top and bottom
values of the range are known as the limits of audibility.
• For the human ear, the lower limit is approximately 20 Hz and the upper limit is 20,000 Hz. In other words, our ears are supposed to be able to hear sound with frequencies that are greater than 20 Hz and less than 20,000 Hz.
• Different people have different ranges of audibility.
• People who are old cannot hear as well as those who are young. The ability of the ear drum to respond to sound decreases with age and the range of audibility becomes very much reduced as the lower limit rises and the upper limit falls.
X-RaysX-Rays• Electrons jump from the
surface of a hot metal –
• Thermionic EmissionThermionic Emission
Accelerated by high voltage they smash into tungsten
The electrons excite orbiting electrons to high energy orbits-see next few slides for details
These fall back emitting high frequency waves.
Most of the electron energy is lost as heat.-about 90%
X-rays very penetrating, fog film, not effected by fields.
High Tension Voltage
Photons
• Bohr first suggested a model for the atom based on many orbits at different energy levels
E1E2
Photons
• If the electron in E1 is excited it can only jump to E2.
E1E2
Photons
• Then the electron falls back. The gap is fixed so the energy it gives out is always the same
E1E2
Photons
• So Max Planck said all energy must come in these packets called photons.
• He came up with a formula for the frequency
E1E2
E2 –E1 = h.f
Where f=frequency
h= Planck’s constant
QuickTime™ and aTIFF (Uncompressed) decompressor
are needed to see this picture.
Albert Einstein
• Uncle Albert was already a Uncle Albert was already a published scientist but the relativity published scientist but the relativity stuff had not set the world alight.stuff had not set the world alight.
• He set his career in real motion He set his career in real motion when he solved a problem and when he solved a problem and started the science of Quantum started the science of Quantum Mechanics that the old world Jew in Mechanics that the old world Jew in him could never come to terms him could never come to terms with.with.
The Problem• If you shine light on the surface of
metals electrons jump off
Polished Sodium MetalPolished Sodium Metal
ee
ee
ee
ee
ee
• Electrons emitted• This is The PHOTOELECTRIC EFFECT
We can prove this with the experiment below
A charged Zinc plate is attached to an Electroscope
When a U.V. lamp is shone on the plate the leaf collapses as all the electrons leave the surface of the zinc
The Photoelectric EffectThe Photoelectric EffectThe more intensity you gave it the more
electrical current was produced However something strange happened
when you looked at frequency
Frequency of light
Electron Energy Newtonian Physics Newtonian Physics
could not explain thiscould not explain this
Einstein’s LawSo we define the Photoelectric effect as:-
Electrons being ejected from the surface of a metal by incident light of a suitable frequency.
Uncle Albert used Plank’s theory that as energy came in packets
A small packet would not give the electron enough energy to leave
Low frequency light had too small a parcel of energy to get the electron free.
Energy of each photon = h.f
Photo-Electric Effect
Frequency of light
Electron Energy
f0=Threshold Frequency
Energy of incident photon =
h.f = h. f0+ KE of electron
Work Function,Energy to release Electron
Energy left over
turnedinto
velocity
Reflection Wave bouncing off a solid object Echo
Refraction
Waves changing speed and direction due to change in density of medium
Frequency stays the same
Hear people across a lake
Diffraction
spreading of a wave around an obstacle or on the emergent side of
a slit.
Better with long wavelength
Sound round cornersSpreading from slit
Interference
Two coherent waves meeting combine wave at any point is the algebraic sum of the two waves
Proves things are waves
Constructive and destructive
PolarisationReduces transverse waves to one plane of oscillation
Difference between transverse and longitudinal
Snow sunglasses