﻿WHAT 
  IS 
  LIGHT? 
  COMPTON 
  225 
  

  

  like 
  a 
  particle. 
  But 
  if 
  X 
  rays, 
  so 
  also 
  all 
  the 
  rest 
  of 
  the 
  family 
  of 
  

   electromagnetic 
  radiations. 
  It 
  would 
  thus 
  seem 
  that 
  by 
  these 
  ex- 
  

   periments 
  Einstein's 
  notion 
  of 
  light 
  as 
  made 
  up 
  of 
  particles 
  is 
  estab- 
  

   lished. 
  

  

  THE 
  PARADOX 
  OF 
  WAVES 
  AND 
  PARTICLES 
  

  

  We 
  thus 
  seem 
  to 
  have 
  satisfactory 
  proof 
  from 
  our 
  interference 
  and 
  

   diffraction 
  experiments 
  that 
  light 
  consists 
  of 
  waves. 
  The 
  photo- 
  

   electric 
  and 
  scattering 
  experiments 
  afford 
  equally 
  satisfactory 
  evi- 
  

   dence 
  that 
  light 
  consists 
  of 
  particles. 
  How 
  can 
  these 
  two 
  apparently 
  

   conflicting 
  concepts 
  be 
  reconciled? 
  

  

  Electron 
  waves. 
  — 
  Before 
  attempting 
  to 
  answer 
  this 
  question 
  let 
  us 
  

   notice 
  that 
  this 
  dilemma 
  applies 
  not 
  only 
  to 
  radiation 
  but 
  also 
  in 
  

   other 
  fundamental 
  fields 
  of 
  physics. 
  When 
  the 
  evidence 
  was 
  growing 
  

   strong 
  that 
  radiation, 
  which 
  we 
  had 
  always 
  thought 
  of 
  as 
  waves, 
  had 
  

   also 
  the 
  properties 
  of 
  particles, 
  L. 
  de 
  Broglie 
  asked, 
  may 
  it 
  not 
  then 
  

   be 
  possible 
  that 
  electrons, 
  which 
  we 
  know 
  as 
  particles, 
  may 
  have 
  the 
  

   properties 
  of 
  waves? 
  An 
  extension 
  of 
  Planck 
  and 
  Einstein's 
  quantum 
  

   theory 
  enabled 
  him 
  to 
  calculate 
  what 
  the 
  wave 
  length 
  corresponding 
  

   to 
  a 
  moving 
  electron 
  should 
  be. 
  In 
  photographs 
  like 
  Plate 
  3, 
  Figure 
  

   1, 
  and 
  Plate 
  4, 
  Figure 
  2, 
  we 
  have 
  ocular 
  evidence 
  that 
  electrons 
  are 
  

   very 
  real 
  particles 
  indeed. 
  Nevertheless, 
  De 
  Broglie's 
  suggestion 
  

   was 
  promptly 
  subjected 
  to 
  experimental 
  test 
  by 
  Davisson 
  and 
  Germer 
  

   at 
  New 
  York, 
  and 
  later 
  by 
  Thomson, 
  Rupp, 
  Kickuchi, 
  and 
  others. 
  

  

  Let 
  us 
  consider 
  Thomson's 
  experiments, 
  which 
  are 
  typical 
  of 
  them 
  

   all. 
  Our 
  crucial 
  evidence 
  for 
  the 
  wave 
  character 
  of 
  light 
  was 
  the 
  fact 
  

   that 
  light 
  could 
  be 
  diffracted 
  by 
  a 
  grating 
  of 
  lines 
  ruled 
  on 
  glass. 
  

   X 
  rays 
  were 
  diffracted 
  in 
  the 
  same 
  way; 
  but 
  before 
  this 
  had 
  been 
  

   shown 
  possible, 
  it 
  was 
  found 
  that 
  X 
  rays 
  could 
  be 
  diffracted 
  by 
  

   the 
  regularly 
  arranged 
  atoms 
  in 
  a 
  crystal. 
  The 
  layers 
  of 
  atoms 
  took 
  

   the 
  place 
  of 
  the 
  lines 
  ruled 
  on 
  glass. 
  Plate 
  5, 
  Figure 
  1, 
  shows 
  how 
  this 
  

   experiment 
  has 
  been 
  done 
  by 
  Hull, 
  at 
  Schenectady. 
  X 
  rays 
  pass 
  

   through 
  a 
  pair 
  of 
  diaphragms 
  and 
  a 
  mass 
  of 
  powdered 
  crystals 
  placed 
  

   at 
  C, 
  and 
  strike 
  a 
  photographic 
  plate 
  at 
  P. 
  Rays 
  diffracted 
  by 
  the 
  

   layers 
  of 
  atoms 
  in 
  the 
  crystal 
  strike 
  at 
  such 
  points 
  as 
  Pi, 
  P2, 
  etc., 
  

   giving 
  rise 
  to 
  a 
  series 
  of 
  rings 
  about 
  the 
  center. 
  If 
  a 
  mass 
  of 
  powdered 
  

   aluminum 
  crystals 
  is 
  placed 
  at 
  C, 
  Hull 
  obtains 
  the 
  photograph 
  shown 
  

   in 
  Plate 
  5, 
  Figure 
  2. 
  You 
  see 
  the 
  central 
  image, 
  and 
  around 
  it 
  the 
  

   diffraction 
  rings. 
  It 
  was 
  this 
  crystal 
  diffraction 
  that 
  first 
  gave 
  con- 
  

   vincing 
  evidence 
  that 
  X 
  rays, 
  like 
  light, 
  consist 
  of 
  waves. 
  

  

  G. 
  P. 
  Thomson 
  has 
  performed 
  a 
  precisely 
  similar 
  experiment 
  with 
  

   electrons. 
  The 
  X-ray 
  beam 
  in 
  the 
  last 
  slide 
  was 
  replaced 
  by 
  a 
  beam 
  

   of 
  cathode 
  electrons, 
  and 
  gold 
  leaf 
  took 
  the 
  place 
  of 
  the 
  aluminum. 
  

   The 
  resulting 
  photograph 
  is 
  shown 
  in 
  Plate 
  5, 
  Figure 
  3. 
  Though 
  it 
  

  

  