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Ch.  15Characteristics  of  Waves

Section  1:    What  Are  Waves?  

• Wave:    disturbance  that  transfers  energy  from  place  to  place.• Energy:    ability  to  do  work  or  cause  change• Medium: material   through  which  a  wave  travels• Mechanical   or  matter  waves:    waves  that  require  a  medium  through  which  to  travel• Vibration:    a  repeated   back   and  forth  or  up  and  down  motion• Transverse  waves:    waves  that  move  the  medium  at  right  angles   to  the  direction  in  which  the  waves   travel

• Crest:    high  part  of    a  transverse  wave• Trough:    low  part  of  a  transverse  wave• Longitudinal  waves:    move   the  medium  parallel   to  the  direction  in  which  the  waves  travel• Compressions:    parts  where  the  coils  (particles)  are  close  together  in  a  longitudinal  wave• Rarefactions:    parts  where  the  coils  are  spread  out,  or  rarif ied  in  a  longitudinal  wave

Waves  and  Energy

• Waves  transfer   energy   from  place  to  place• Example:    think  about  a   raft  in  water-­‐ a  wave  that  disturbs   the  surface  of   the  water  also   will  disturb   the  raft.    The  wave’s  energy  lifts   the  heavy  raft  as  the  wave  passes  unit   it.    But   the  disturbance  caused  by  the  wave  is   temporary.    After   the  wave  passes,   the  water  is   calm  again  and   the  raft  stops  bobbing.

What  carries  waves?• Most  waves  need  a  material  to  travel  through  (a  medium),  these  are  called  mechanical  or  matter  waves• Sound  waves  travel  through  air• Water  waves   travel  along   the  surface  of  water• A  wave  can  travel  through  a  rope• They  can  travel  through  solids,   liquids,   or  gases

• Some  waves  can  travel  through  empty  space,  called  electromagnetic  waves• Light  from  the  sun• Anything    on  what   is   called  the  electromagnetic  spectrum

How  Do  Waves  Transfer  Energy?

• Although  mechanical  waves  need  a  medium  to  travel  through                  they  do  not  move  the  medium  itself,  they  just  move  through  it• Why?

• All  mediums  are   made   of  tiny  particles.    When  a  wave   enters  a   medium,  it  transfers  energy   to  the  medium’s  particles.    The   particles  bump  into  each  other,  passing  the  wave’s   energy   along.

• Similar   to  how  food  is  passed  around  a  table  during  dinner.    The  food  being  passed  around  the  table  represents  the  energy.    The   people  stay  in  their  seats  just  like   the  particles  of  the  medium.

What  Causes  Waves?

• ENERGY  is  required  to  make  all  waves• Mechanical  waves  are  produced  when  a  source  of  energy  causes  a  medium  to  vibrate• Moving  objects  have  energy  and  can  transfer  energy  to  medium  and  cause  waves• Example:    you  can  make  waves  by  putting  your  finger   into  a  bowl  of  water• Example:    a  motorboat  moves  through  water  and   transfers  it’s   energy  to  the  water  making  waves

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Types  of  Waves

• There   are   two  kinds  of  mechanical  waves• Classified   by  the  wave  they  move

Transverse  waves

• They  move   the   medium   at  a  right   angle   to   the  direction   they  are  moving• Transverse   means   across• As   the  wave   moves   the   particles   of  the  medium   move   across   or  at   a   right   and  to,  the  direction   of  the   wave

• Examples:    water   waves,   shaking   a  rope   up  and   down

Longitudinal  wave

• Move  the  medium  parallel  to  the  direction  of  the  wave’s  motion• A  Slinky  produces  this  kind  of  wave  when  stretched  • Sound  waves  cause  air  particles  to  move  back  and  forth

Section  2:    Properties  of  Waves

• Amplitude:    maximum  distance  that  the  particles  of  the  medium  carrying  the   wave  move  away  from  their  rest  position• Wavelength:    distance  between  two  corresponding  parts  of  a  wave• Frequency:    number  of  complete  waves  that  pass  a  given  point  in  a  certain  amount  of  time.• Hertz:    unit  that  frequency  is  measured  in

Properties  of  Waves

• The  basic  properties  of  waves  are  amplitude,  wavelength,  frequency,  and  speed

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Amplitude

• Distance  the  medium  rise  or   lowers  depends   on  the  amplitude

• The  more  energy  a  wave  has   the  greater  the  amplitude

• Amplitude   of  a  transverse  wave• Maximum   distance  the  wave  moves   up  or  down  from  rest  position

• Amplitude   of  a  longitudinal   wave• How  compressed  or  raref ied  the  medium  becomes• High  energy   waves   causes   more  compression  and  rarefaction

Wavelength

• Distance  between   two  corresponding  parts  of  a  wave• Ex.  Distance  between  two  crests  or  two   troughs

• But  it  just  has  to  be  the  same  point  on  the  next  wave

Frequency

• They  number  of  waves  that  pass  a  certain  point  in  a  given  amount  of  time• Watch  a  reference  point  and  count  how  many  crests  pass   in  a  minute

• A  wave   that  has  that  occurs  every  second  has  a  frequency  of  1  Hertz,  if  2  occur  in  a  second  its  frequency  is  2  Hertz

Transverse  wave:    label  a  wavelength  and  amplitude

Speed

• How  quickly  a  wave  moves  through  a  medium• Distance/time

Relating  the  properties  with  an  equation

• Speed=wavelength   x  Frequency• If  the  medium  does  not  change  the  speed   for  a  wave  is   constant

• So  if  you  multiply  wavelength  by  frequency  you  should  always  get  the  same  speed• If  frequency  is   increased,   then   the  wavelength  decreases

• Inverse   relationship

• Frequency  =  speed/wavelength• Wavelength  =  speed/frequency

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Section  3:    Interactions  of  Waves• Reflection:    when  an  object  or  wave  hits  a  surface  through  which  it  cannot  pass,  it  bounces  back• Law  of  reflection:     states  that  the  angle  of  incidence  equals  the  angle   of  ref lection• Refraction:    bending  of  waves  due  to  a  change  in  speed• Diffraction:    when  a  wave  moves   around  a  barrier  or  through  an  opening  in  a  barrier,  it  bends  and  spreads  out

• Interference:    interaction  between  two  waves  that  meet• Constructive  interference:    when  waves  combine  to  make  a  wave  with  a  larger  amplitude• Destructive  interference:    when  two  waves  combine  to  make  a  wave  with  a  smaller  amplitude• Standing  wave:    a  wave   that  appears  to  stand  in  one  place,  even  though  it  is  really   two  waves  interfering  as  they  pass  through  each  other

• Nodes: points  of  zero  amplitude  on  a  standing  wave• Antinodes:    points  of  maximum  amplitude  on  a  standing  wave• Resonance:    an  increase  in  the  amplitude  of  a  vibration  that  occurs  when  external  vibrations  match  an  object’s  natural  frequency  

Interactions  of  Waves

• There   are   five  types  of  interactions  with  waves:    reflection,  refraction,  diffraction,  interference,  and  standing  waves

Reflection

• Wave  or  object  hits  a  surface  it  cannot  pass  through  and  bounces  back• Examples• An  echo  is   created  when  sound  waves  are  reflected• Mirror   images  are  created  when   light  waves  are  reflected

Law  of  Reflection

• The  angle  of  incidence  (between  ingoing  wave  and  normal)  must  equal  the  angle  of  reflection  (between  reflected  wave   and  normal)

Refraction

• When   a  wave  enters   a  new  medium   at  a  angle,   one  side   of   the  wave  changes   speed   before   the  other,   causing   the   wave   to  bend

• This   only   occurs   if   the   wave  enters   the  new   medium   at  an  angle• Bending   occurs   because   the  two  sides   of  the  wave   are   traveling   at   dif ferent  speeds

• The  type  of  medium  determines  the  speed  the  wave  is  traveling

• Example:    rainbows   form  because   the   light   waves   bend   as   they  pass   from  water   into   air

• Have  you  ever   tried   to   reach   for   something   in  water   and   found   it  wasn’t  where   you   thought?• When   white   light   enters   water   dif ferent   colors  of  the   light   bend  by   dif ferent  amounts

Diffraction

• When  waves  bend  to  go  around  a  barrier  or  through  a  hole• After   the  passes   the  barrier   it   spreads  back  out• The  smaller   the  opening,   the  more   the  wave  has   to  bend• Examples:    water  waves  bend  around   islands   and  sound  waves  bend  around  corners  or  through  door  openings  

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Interference

• When  two  waves  meet• Examples• Water  waves  crash  into  each  other  in   the  ocean• Sound  waves  from  speakers  

Constructive  interference

• combine  in  a  way  to  make  larger  amplitudes• When  two  crests  overlap   they  make  a  higher  crest• When  two   troughs  overlap  they  make  a  deeper  trough• After   the  waves  pass  each  other  they  return   to  normal• Example:    this  causes   the  sounds   to  be   louder

Destructive  interference

• combine  in  a  way  to  make  smaller  amplitudes• Crest  of  one  wave  meets  the   trough  of  another  wave

• If  the  crest  “wins”  part  of  it  remains  and  if  the  trough  “wins”  part  of  it  remains• If  the  original  waves   had  equal  amplitudes  then  they  can  completely  cancel   out

• Example:    sounds  will  sound  quieter  or  may  even  cancel  out  completely.    

Standing  Waves

• Looks  like  the  wave  is  standing  still,  but  in  fact  its  two  waves  interfering  as  they  pass  through  each  other• The  incoming  and   reflected  wave  has  just   the  right  frequency

• Parts  of  standing  wave• Nodes:    points  of   zero  amplitude   (where  wave  meets  rest  position)

• Antinodes:    points   of  maximum  amplitude   (crests  or  troughs)

Resonance

• An  increase  in  the  amplitude  of  a  vibration  that  occurs  when  external  vibrations  match  an  object’s  natural  frequency.• Musical   instruments   use   this   to  produce  stronger,   clearer   sounds