﻿WHAT 
  IS 
  LIGHT? 
  COMPTON 
  223 
  

  

  decrease 
  in 
  frequency 
  of 
  the 
  scattered 
  X 
  rays, 
  just 
  as 
  the 
  experiments 
  

   show. 
  In 
  fact, 
  the 
  theory 
  is 
  so 
  definite 
  that 
  it 
  is 
  possible 
  to 
  calculate 
  

   just 
  how 
  great 
  a 
  change 
  in 
  frequency 
  should 
  occur, 
  and 
  the 
  calculation 
  

   is 
  found 
  to 
  correspond 
  accurately 
  with 
  the 
  experiments. 
  

  

  Trailing 
  a 
  photon. 
  — 
  If 
  this 
  explanation 
  is 
  the 
  correct 
  one 
  it 
  should, 
  

   however, 
  be 
  possible 
  to 
  find 
  the 
  electrons 
  which 
  recoil 
  from 
  the 
  impact 
  

   of 
  the 
  X-ray 
  particles. 
  Before 
  this 
  theory 
  of 
  the 
  origin 
  of 
  scattered 
  

   X 
  rays 
  was 
  suggested, 
  no 
  such 
  recoiling 
  electrons 
  had 
  ever 
  been 
  

   noticed. 
  Within 
  a 
  few 
  months 
  after 
  its 
  proposal, 
  however, 
  C. 
  T. 
  R. 
  

   Wilson 
  succeeded 
  in 
  photographing 
  the 
  tracks 
  left 
  when 
  electrons 
  in 
  

  

  INCIDENT 
  QUANTUM 
  

  

  Figure 
  5.— 
  Recoil 
  of 
  an 
  electron. 
  When 
  an 
  incident 
  X-ray 
  photon 
  glances 
  from 
  an 
  electron, 
  the 
  

   electron 
  recoils 
  from 
  the 
  impact, 
  taking 
  part 
  of 
  the 
  photon's 
  energy 
  

  

  air 
  recoil 
  from 
  the 
  X 
  rays 
  which 
  they 
  scatter. 
  Plate 
  4, 
  Figure 
  2, 
  

   shows 
  one 
  of 
  his 
  typical 
  photographs. 
  The 
  X 
  rays 
  here 
  are 
  going 
  

   from 
  left 
  to 
  right. 
  At 
  top 
  and 
  bottom 
  will 
  be 
  seen 
  the 
  long 
  trails 
  

   left 
  by 
  two 
  photoelectrons, 
  which 
  as 
  we 
  have 
  seen 
  take 
  up 
  the 
  whole 
  

   energy 
  of 
  a 
  photon. 
  In 
  between 
  are 
  a 
  number 
  of 
  shorter 
  trails, 
  all 
  

   with 
  their 
  tails 
  toward 
  the 
  X-ray 
  tube. 
  These 
  are 
  the 
  electrons 
  

   which 
  have 
  been 
  struck 
  by 
  flying 
  X-ray 
  photons. 
  Some 
  have 
  been 
  

   struck 
  squarely, 
  and 
  are 
  knocked 
  straight 
  ahead. 
  Others 
  have 
  

   received 
  only 
  a 
  glancing 
  blow, 
  and 
  have 
  recoiled 
  at 
  an 
  angle. 
  Thus 
  

   we 
  have 
  observed 
  not 
  only 
  the 
  loss 
  in 
  energy 
  of 
  the 
  deflected 
  photons, 
  

  

  