﻿WHAT 
  IS 
  LIGHT? 
  — 
  COMPTON 
  

  

  217 
  

  

  That 
  these 
  rays 
  are 
  of 
  the 
  same 
  nature 
  as 
  hght 
  is 
  shown 
  by 
  the 
  fact 
  

   that 
  we 
  have 
  found 
  it 
  possible 
  to 
  reflect 
  and 
  refract 
  them, 
  to 
  polarize 
  

   and 
  diffract 
  them. 
  They 
  are 
  indeed 
  light 
  of 
  ten 
  thousand 
  times 
  

   shorter 
  wave 
  length. 
  

  

  One 
  of 
  the 
  most 
  important 
  properties 
  of 
  X 
  rays 
  is 
  their 
  ability 
  to 
  

   ionize 
  air 
  and 
  make 
  it 
  electrically 
  conducting. 
  Such 
  ionization 
  can 
  

   also 
  be 
  produced 
  by 
  the 
  gamma 
  rays 
  from 
  radium. 
  Whereas, 
  how- 
  

   ever, 
  X 
  rays 
  may 
  be 
  half 
  absorbed 
  in 
  an 
  inch 
  of 
  water, 
  it 
  takes 
  a 
  foot 
  

   of 
  water 
  to 
  absorb 
  half 
  of 
  the 
  gamma 
  rays 
  from 
  radium, 
  correspond- 
  

   ing 
  to 
  the 
  much 
  shorter 
  wave 
  length 
  of 
  the 
  radioactive 
  rays. 
  

  

  But 
  the 
  end 
  is 
  not 
  yet. 
  There 
  exists 
  a 
  kind 
  of 
  highly 
  penetrating 
  

   radiation 
  which 
  is 
  especially 
  prominent 
  at 
  high 
  altitudes, 
  and 
  is 
  sup- 
  

   posed 
  to 
  come 
  from 
  some 
  source 
  outside 
  the 
  earth. 
  These 
  cosmic 
  rays, 
  

   as 
  they 
  are 
  called, 
  will 
  penetrate 
  10 
  or 
  20 
  feet 
  of 
  water 
  before 
  they 
  are 
  

  

  Electric 
  cr 
  T^adio 
  Bays 
  

  

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  Ultra- 
  

  

  Qawmo 
  

   T?avs 
  

  

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  10^^. 
  

  

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  n 
  w~ 
  

  

  Watie 
  Length 
  ni',C-m. 
  

  

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  10 
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  10 
  

  

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  V//A 
  

  

  Broadcasting 
  Heat 
  Tlays 
  X-rajis 
  Cosmic 
  

  

  Band 
  -Ra^js 
  

  

  Complete 
  Spectrum 
  of 
  Electroma^vetic 
  'Radiation, 
  l9^'=f 
  (t^tin^^M) 
  

  

  Figure 
  2. 
  — 
  Complete 
  spectrum 
  of 
  electromagnetic 
  radiation 
  on 
  a 
  logarithmic 
  scale. 
  Visible 
  light 
  is 
  

   only 
  a 
  small 
  but 
  very 
  important 
  part 
  of 
  this 
  spectrum 
  

  

  half 
  absorbed. 
  It 
  is 
  possible 
  that 
  these 
  rays 
  are 
  like 
  cathode 
  rays, 
  

   rather 
  than 
  X 
  rays, 
  though 
  they 
  are 
  usually 
  thought 
  to 
  be 
  of 
  the 
  latter 
  

   type. 
  

  

  In 
  Figure 
  2 
  we 
  see 
  graphically 
  how 
  these 
  different 
  rays 
  are 
  related 
  

   to 
  each 
  other. 
  At 
  the 
  extreme 
  left 
  I 
  have 
  arbitrarily 
  started 
  the 
  

   spectrum 
  at 
  a 
  wave 
  length 
  of 
  18 
  kilometers, 
  which 
  is 
  the 
  wave 
  length 
  

   of 
  certain 
  trans-Atlantic 
  wireless 
  signals. 
  There 
  is 
  no 
  reason 
  why 
  

   longer 
  waves 
  could 
  not 
  be 
  produced 
  if 
  desired. 
  The 
  electric 
  waves 
  

   continue 
  in 
  an 
  unbroken 
  spectrum 
  down 
  to 
  0.1 
  mm., 
  rays 
  recently 
  

   studied 
  at 
  Cleveland 
  by 
  the 
  late 
  Doctor 
  Nichols 
  and 
  Mr. 
  Tear. 
  Over- 
  

   lapping 
  these 
  electric 
  rays 
  are 
  the 
  heat 
  waves, 
  which 
  have 
  been 
  ob- 
  

   served 
  from 
  about 
  0.03 
  cm. 
  to 
  0.00003 
  cm., 
  including 
  the 
  whole 
  of 
  the 
  

   visible 
  region. 
  The 
  heat 
  rays 
  in 
  turn 
  are 
  overlapped 
  by 
  the 
  ultra- 
  

   violet 
  rays, 
  produced 
  by 
  electric 
  discharges; 
  and 
  these 
  reach 
  well 
  into 
  

   the 
  region 
  described 
  as 
  X 
  rays. 
  Beyond 
  these 
  are 
  in 
  turn 
  the 
  gamma 
  

   rays 
  and 
  the 
  cosmic 
  rays. 
  Thus 
  over 
  a 
  range 
  of 
  wave 
  lengths 
  of 
  from 
  

   2 
  X 
  10~^^ 
  cm. 
  to 
  2X10'^ 
  cm. 
  there 
  is 
  found 
  to 
  be 
  a 
  continuous 
  spectrum 
  

   of 
  radiations, 
  of 
  which 
  visible 
  light 
  occupies 
  only 
  a 
  very 
  narrow 
  band. 
  

  

  The 
  great 
  breadth 
  of 
  this 
  wave-length 
  range 
  will 
  perhaps 
  be 
  better 
  

   appreciated 
  if 
  we 
  expand 
  the 
  scale 
  until 
  the 
  wave 
  of 
  a 
  cosmic 
  ray 
  has 
  a 
  

   length 
  equal 
  to 
  the 
  thickness 
  of 
  a 
  post 
  card. 
  The 
  longest 
  wireless 
  

   wave 
  would 
  on 
  this 
  scale 
  extend 
  from 
  here 
  to 
  the 
  nearest 
  fixed 
  star. 
  

  

  