2. What is a wave?
To answer this question, we will look only atTo answer this question, we will look only at
mechanical waves. Mechanical waves require aMechanical waves require a
medium through which to move. Commonmedium through which to move. Common
mechanical waves are:mechanical waves are: water waveswater waves andand soundsound
waveswaves..
Electromagnetic waves (light, x, gamma, radio,(light, x, gamma, radio,
UV waves) doUV waves) do not need a medium to travelneed a medium to travel
through.through. However, early scientists believed thatHowever, early scientists believed that
there was something calledthere was something called ether that permeatedthat permeated
space which allowed light to travel through it!space which allowed light to travel through it!
A mechanical wave is a disturbance thatA mechanical wave is a disturbance that
transfers energy through a medium.through a medium.
Water waves are the easiest to visualize, think ofWater waves are the easiest to visualize, think of
a rock thrown into a pond. The waves movea rock thrown into a pond. The waves move
outwards from where the rock hit the water.outwards from where the rock hit the water.
3. Wave PropertiesWave Properties
The speed of a wave, through a specificThe speed of a wave, through a specific
medium, depends only on the medium NOT themedium, depends only on the medium NOT the
size of a wave.size of a wave.
Think of a very large rock thrown into a pond,Think of a very large rock thrown into a pond,
the waves moving outwards move at the samethe waves moving outwards move at the same
speed as waves created by a small rock. So,speed as waves created by a small rock. So,
thethe amplitude of the wave does not affect itsof the wave does not affect its
speed.speed.
Sound waves are similar: “loud” sounds travel noSound waves are similar: “loud” sounds travel no
faster than “quiet” sounds in a specific mediumfaster than “quiet” sounds in a specific medium
(usually air).(usually air).
4. A typical water wave has a sinusoidal shape (side profile).A typical water wave has a sinusoidal shape (side profile).
The drawing shows aThe drawing shows a continuous wave which could be createdwhich could be created
by a vibrating source in a ripple tank. The video link shows twoby a vibrating source in a ripple tank. The video link shows two
waves sources (which cause a wave interference pattern).waves sources (which cause a wave interference pattern).
http://www.youtube.com/watch?v=ovZkFMuxZNc&feature=relatedhttp://www.youtube.com/watch?v=ovZkFMuxZNc&feature=related
The diagram labels the parts of a wave. This wave represents aThe diagram labels the parts of a wave. This wave represents a
snapshot in time and the horizontal axis represents a lengthsnapshot in time and the horizontal axis represents a length
measurement, andmeasurement, and λ (lambda) represents wavelength.(lambda) represents wavelength. λ cancan
be measured from many different points, as long as thebe measured from many different points, as long as the
measurement is from the start of one wave to the start of themeasurement is from the start of one wave to the start of the
very next wave.very next wave.
C o n t i u o u s W a v e
Amplitude
D O M
U p
D o w n T r o u g h
C r e s t
5. The Fundamental Wave Equation
All waves (mechanical & electromagnetic) obeyAll waves (mechanical & electromagnetic) obey
this equation:this equation: v = f x λ ,, herehere v is the speedis the speed
(m/s),(m/s), f is frequency (Hz) andis frequency (Hz) and λ is wavelengthis wavelength
(m).(m).
Relating this equation to waves created in aRelating this equation to waves created in a
ripple tank, if the frequency of the wave sourceripple tank, if the frequency of the wave source
is increased (is increased (f is larger), thenis larger), then λ must decreasemust decrease
sincesince v is constant (as long as the water depthis constant (as long as the water depth
does not change).does not change).
In the ripple tank, waves will travelIn the ripple tank, waves will travel faster in deep
water than in shallow water. We treat changesthan in shallow water. We treat changes
in depth as if there were a change in medium –in depth as if there were a change in medium –
so there must be a new wave speed.so there must be a new wave speed. Note, theNote, the
frequency of the wave does not change as it
moves into a new medium..
6. Two types of waves:Two types of waves:
Transverse andand
Longitudinal
For aFor a transverse wave,,
the motion of the mediumthe motion of the medium
isis ⊥ to the DOM of theto the DOM of the
wave – the boat moveswave – the boat moves
up and down.up and down.
For aFor a longitudinal wave,,
the motion of the mediumthe motion of the medium
isis // to DOM of the waveto DOM of the wave
(sound wave)(sound wave)
For both types of waves,
the medium does not
move with the wave.
D O M
A lo n g it u d in a l w a v e
t r a v e llin g d o w n a s lin k y
T r a n s v e r s e W a v e
W a v e
D O M
7. Practice Questions
Nelson TB:Nelson TB:
Page 389 #1-3Page 389 #1-3
Page 391 #1, 3, 5, 6Page 391 #1, 3, 5, 6
From the McGraw-Hill TB:
A wave with an amplitude of 50.0 cm travels downA wave with an amplitude of 50.0 cm travels down
a 8.0 m spring in 4.5s. The person who createsa 8.0 m spring in 4.5s. The person who creates
the wave moves her hand through 4 cycles in 1the wave moves her hand through 4 cycles in 1
second. What is the wavelength? (second. What is the wavelength? (Ans. 0.44 m))
Brief video on transverse and longitudinal wavesBrief video on transverse and longitudinal waves
https://www.youtube.com/watch?v=jAXx0018QCchttps://www.youtube.com/watch?v=jAXx0018QCc
http://www.youtube.com/watch?v=7cDAYFTXq3Ehttp://www.youtube.com/watch?v=7cDAYFTXq3E
