﻿WHAT 
  IS 
  LIGHT? 
  COMPTON 
  

  

  219 
  

  

  The 
  same 
  experiment 
  may 
  be 
  performed 
  with 
  a 
  beam 
  of 
  hght. 
  In 
  

   Plate 
  2, 
  Figure 
  1, 
  is 
  shown 
  a 
  set 
  of 
  some 
  200 
  vertical 
  lines. 
  If 
  these 
  

   lines 
  are 
  photographed 
  onto 
  a 
  lantern 
  slide, 
  they 
  form 
  a 
  grid 
  through 
  

   which 
  a 
  beam 
  of 
  light 
  may 
  be 
  made 
  to 
  pass. 
  The 
  upper 
  part 
  of 
  

   Plate 
  2, 
  Figure 
  2, 
  shows 
  a 
  beam 
  of 
  light 
  projected 
  onto 
  a 
  photographic 
  

   plate. 
  The 
  middle 
  part 
  of 
  the 
  figure 
  shows 
  the 
  same 
  beam 
  of 
  light, 
  

   but 
  this 
  time 
  projected 
  through 
  such 
  a 
  lantern 
  slide 
  grid 
  having 
  about 
  

   100 
  lines 
  to 
  the 
  inch. 
  The 
  original 
  spot 
  of 
  light 
  is 
  now 
  split 
  into 
  three, 
  

   a 
  bright 
  one 
  in 
  the 
  center, 
  the 
  direct 
  ray, 
  and 
  a 
  diffracted 
  ray 
  on 
  either 
  

   side. 
  It 
  is 
  just 
  as 
  in 
  the 
  case 
  of 
  the 
  mercury 
  ripples 
  passing 
  through 
  

   the 
  grid. 
  

  

  If 
  this 
  is 
  really 
  a 
  case 
  of 
  the 
  diffraction 
  of 
  waves, 
  as 
  we 
  have 
  sup- 
  

   posed, 
  if 
  a 
  grating 
  with 
  lines 
  closer 
  together 
  is 
  used, 
  the 
  separation 
  

   between 
  the 
  diffracted 
  images 
  should 
  be 
  correspondingly 
  greater. 
  

   The 
  lower 
  part 
  of 
  Plate 
  2, 
  Figure 
  2, 
  shows 
  our 
  beam 
  of 
  light 
  projected 
  

   this 
  time 
  through 
  a 
  grid 
  photographed 
  with 
  about 
  300 
  lines 
  to 
  the 
  

   inch. 
  The 
  separation 
  of 
  the 
  diffracted 
  beams 
  is 
  now 
  much 
  greater. 
  

  

  Figure 
  3. 
  — 
  Apparatus 
  for 
  diSracting 
  X 
  rays 
  from 
  a 
  ruled 
  reflection 
  grating 
  

  

  When 
  these 
  diffracted 
  images 
  are 
  thrown 
  on 
  a 
  screen, 
  one 
  can 
  see 
  that 
  

   their 
  outer 
  edges 
  are 
  red 
  and 
  their 
  inner 
  edges 
  blue. 
  This 
  means 
  that 
  

   red 
  light 
  is 
  of 
  the 
  greater 
  wave 
  length. 
  In 
  fact 
  we 
  could 
  easily, 
  from 
  

   this 
  experiment, 
  tell 
  what 
  the 
  wave 
  length 
  of 
  light 
  is 
  — 
  the 
  distance 
  

   from 
  the 
  central 
  image 
  to 
  the 
  diffracted 
  image 
  is 
  to 
  the 
  distance 
  from 
  

   the 
  grating 
  to 
  the 
  screen 
  as 
  the 
  wave 
  length 
  of 
  the 
  light 
  is 
  to 
  the 
  dis- 
  

   tance 
  between 
  the 
  lines 
  on 
  the 
  grating. 
  When 
  one 
  carries 
  through 
  

   the 
  calculation, 
  he 
  finds 
  that 
  the 
  wave 
  length 
  of 
  light 
  is 
  about 
  one 
  

   fifty-thousandth 
  of 
  an 
  inch. 
  

  

  If 
  we 
  can 
  rely 
  on 
  such 
  a 
  test, 
  light 
  must 
  consist 
  of 
  waves. 
  

  

  Diffraction 
  of 
  X 
  rays. 
  — 
  Precisely 
  similar 
  experiments 
  can, 
  however, 
  

   be 
  done 
  with 
  X 
  rays. 
  In 
  place 
  of 
  a 
  projection 
  lantern 
  we 
  must, 
  

   however, 
  use 
  an 
  X-ray 
  tube 
  and 
  a 
  pair 
  of 
  slits 
  as 
  shown 
  in 
  Figure 
  3. 
  

   The 
  lantern 
  slide 
  with 
  the 
  lines 
  on 
  it 
  is 
  replaced 
  by 
  a 
  polished 
  mirror 
  

   on 
  which 
  lines 
  are 
  ruled 
  50 
  to 
  the 
  millimeter. 
  The 
  resulting 
  photo- 
  

   graph 
  is 
  shown 
  in 
  Plate 
  2, 
  Figure 
  3. 
  When 
  the 
  ruled 
  mirror 
  is 
  with- 
  

  

  