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  ANNUAL 
  EEPORT 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  of 
  metal. 
  Here 
  the 
  photon 
  gives 
  up 
  its 
  energy 
  to 
  one 
  of 
  the 
  electrons 
  

   of 
  which 
  the 
  metal 
  is 
  composed, 
  and 
  throws 
  it 
  out 
  with 
  an 
  energy 
  

   of 
  motion 
  equal 
  to 
  that 
  of 
  the 
  first 
  electron. 
  

  

  In 
  this 
  way 
  Einstein 
  was 
  able 
  to 
  account 
  in 
  a 
  very 
  satisfactory 
  

   way 
  for 
  the 
  phenomenon 
  of 
  the 
  ejection 
  of 
  electrons 
  by 
  light 
  and 
  

   X 
  rays. 
  

  

  How 
  X 
  rays 
  are 
  scattered. 
  — 
  Even 
  more 
  direct 
  evidence 
  that 
  light 
  

   consists 
  of 
  particles 
  has 
  come 
  from 
  a 
  study 
  of 
  scattered 
  X 
  rays. 
  

   If 
  a 
  piece 
  of 
  paper 
  is 
  held 
  in 
  the 
  light 
  of 
  a 
  lamp, 
  the 
  paper 
  scatters 
  

   light 
  from 
  the 
  lamp 
  into 
  our 
  eyes. 
  In 
  the 
  same 
  way, 
  if 
  the 
  lamp 
  were 
  

   an 
  X-ray 
  tube, 
  the 
  paper 
  would 
  scatter 
  X 
  rays 
  into 
  our 
  eyes. 
  If 
  

   light 
  and 
  X 
  rays 
  are 
  waves, 
  scattered 
  X 
  rays 
  are 
  like 
  an 
  echo. 
  When 
  

   one 
  whistles 
  in 
  front 
  of 
  a 
  wall, 
  the 
  echo 
  comes 
  back 
  with 
  the 
  same 
  

   pitch 
  as 
  the 
  original 
  sound. 
  This 
  must 
  be 
  so, 
  for 
  each 
  wave 
  of 
  the 
  

   sound 
  is 
  reflected 
  from 
  the 
  wall, 
  as 
  many 
  waves 
  return 
  as 
  strike, 
  and 
  

   the 
  frequency 
  or 
  pitch 
  of 
  the 
  echoed 
  wave 
  is 
  the 
  same 
  as 
  that 
  of 
  the 
  

   original 
  wave. 
  In 
  the 
  case 
  of 
  scattered 
  X 
  rays, 
  the 
  echo 
  should 
  simi- 
  

   larly 
  be 
  thrown 
  back 
  by 
  the 
  electrons 
  in 
  the 
  scattering 
  material, 
  

   and 
  should 
  likewise 
  have 
  the 
  same 
  pitch 
  or 
  frequency 
  as 
  the 
  incident 
  

   rays. 
  

  

  We 
  can 
  measure 
  the 
  pitch, 
  or 
  what 
  amounts 
  to 
  the 
  same 
  thing, 
  the 
  

   wave 
  length 
  of 
  a 
  beam 
  of 
  scattered 
  X 
  rays, 
  using 
  the 
  apparatus 
  shown 
  

   in 
  Plate 
  3, 
  Figure 
  2. 
  Rays 
  from 
  the 
  target 
  T 
  of 
  the 
  X-ray 
  tube 
  were 
  

   scattered 
  by 
  a 
  block 
  of 
  carbon 
  at 
  R, 
  and 
  the 
  wave 
  length 
  of 
  the 
  echoed 
  

   rays 
  was 
  measured 
  by 
  an 
  X-ray 
  spectrometer. 
  By 
  swinging 
  the 
  X- 
  

   ray 
  tube 
  in 
  line 
  with 
  the 
  slits, 
  it 
  was 
  possible 
  to 
  get 
  a 
  direct 
  com- 
  

   parison 
  with 
  the 
  wave 
  length 
  of 
  the 
  original 
  rays. 
  

  

  Plate 
  4, 
  Figure 
  1, 
  shows 
  the 
  result 
  of 
  the 
  experiment. 
  Above 
  is 
  

   plotted 
  the 
  spectrum 
  of 
  the 
  original 
  X-ray 
  beam. 
  Below 
  is 
  shown 
  the 
  

   spectrum 
  of 
  the 
  X 
  rays 
  scattered 
  in 
  three 
  different 
  directions. 
  A 
  part 
  

   of 
  the 
  scattered 
  rays 
  is 
  of 
  the 
  original 
  wave 
  length; 
  but, 
  as 
  you 
  see, 
  

   most 
  of 
  the 
  rays 
  are 
  increased 
  in 
  wave 
  length. 
  This 
  would 
  corres- 
  

   pond 
  to 
  a 
  lower 
  pitch 
  for 
  the 
  echo 
  than 
  for 
  the 
  original 
  sound. 
  

  

  As 
  we 
  have 
  seen, 
  this 
  change 
  in 
  wave 
  length 
  is 
  contrary 
  to 
  the 
  

   predictions 
  of 
  the 
  wave 
  theory. 
  If 
  we 
  take 
  Einstein's 
  idea 
  of 
  X-ray 
  

   particles, 
  however, 
  we 
  find 
  a 
  simple 
  explanation 
  of 
  the 
  effect. 
  On 
  this 
  

   view, 
  we 
  may 
  suppose 
  that 
  each 
  photon 
  of 
  the 
  scattered 
  X 
  rays 
  is 
  

   deflected 
  by 
  a 
  single 
  electron, 
  Figure 
  5. 
  Picture 
  a 
  golf 
  ball 
  bouncing 
  

   from 
  a 
  football. 
  A 
  part 
  of 
  the 
  golf 
  ball's 
  energy 
  is 
  spent 
  in 
  setting 
  

   the 
  football 
  in 
  motion. 
  Thus 
  the 
  golf 
  ball 
  bounces 
  off 
  having 
  less 
  

   energy 
  than 
  when 
  it 
  struck. 
  In 
  the 
  same 
  way 
  the 
  electron 
  from 
  which 
  

   the 
  X-ray 
  photon 
  bounces 
  will 
  recoil, 
  taking 
  part 
  of 
  the 
  photon's 
  

   energy, 
  and 
  the 
  deflected 
  photon 
  will 
  have 
  less 
  energy 
  than 
  before 
  it 
  

   struck 
  the 
  electron. 
  This 
  reduction 
  in 
  energy 
  of 
  the 
  X-ray 
  photon 
  

   corresponds, 
  according 
  to 
  Planck's 
  original 
  quantum 
  theory, 
  to 
  a 
  

  

  