﻿WHAT 
  IS 
  LIGHT? 
  

  

  By 
  Arthur 
  H. 
  Compton 
  

  

  [With 
  5 
  plates] 
  

  

  As 
  long 
  ago 
  as 
  the 
  seventeenth 
  century, 
  Newton 
  defended 
  the 
  view 
  

   that 
  light 
  consists 
  of 
  streams 
  of 
  little 
  particles, 
  shot 
  with 
  tremendous 
  

   speed 
  from 
  a 
  candle 
  or 
  the 
  sun 
  or 
  any 
  other 
  source 
  of 
  light. 
  At 
  the 
  

   dawn 
  of 
  the 
  nineteenth 
  century, 
  however, 
  experiments 
  were 
  performed 
  

   which 
  were 
  thought 
  to 
  give 
  positive 
  evidence 
  that 
  light 
  consists 
  of 
  

   waves. 
  Maxwell 
  interpreted 
  them 
  as 
  electromagnetic 
  waves, 
  and 
  in 
  

   such 
  terms 
  we 
  have 
  ever 
  since 
  been 
  explaining 
  light 
  rays, 
  X 
  rays, 
  and 
  

   radio 
  rays. 
  We 
  have 
  measured 
  the 
  length 
  of 
  the 
  waves, 
  their 
  fre- 
  

   quency 
  and 
  other 
  characteristics, 
  and 
  have 
  felt 
  that 
  we 
  know 
  them 
  

   intimately. 
  Recently, 
  however, 
  a 
  group 
  of 
  electrical 
  effects 
  of 
  light 
  

   has 
  been 
  discovered 
  for 
  w^hich 
  the 
  idea 
  of 
  light 
  waves 
  suggests 
  no 
  

   explanation, 
  but 
  whose 
  interpretation 
  is 
  obvious 
  accordmg 
  to 
  a 
  

   modified 
  form 
  of 
  Newton's 
  old 
  theory 
  of 
  light 
  projectiles. 
  

  

  REVIEW 
  OF 
  THE 
  VARIOUS 
  ELECTROMAGNETIC 
  RADIATIONS 
  

  

  When 
  the 
  physicist 
  speaks 
  of 
  light 
  he 
  thinks 
  not 
  only 
  of 
  those 
  radia- 
  

   tions 
  which 
  affect 
  the 
  eye. 
  He 
  refers 
  rather 
  to 
  a 
  wide 
  range 
  of 
  radia- 
  

   tions, 
  similar 
  to 
  visible 
  light 
  in 
  essential 
  nature, 
  but 
  differing 
  in 
  the 
  

   quality 
  described 
  variously 
  by 
  the 
  terms 
  color, 
  wave 
  length, 
  or 
  fre- 
  

   quency. 
  

  

  At 
  one 
  end 
  of 
  this 
  series 
  of 
  radiations 
  are 
  the 
  wireless, 
  or 
  radio 
  

   rays, 
  with 
  which 
  in 
  recent 
  years 
  w^e 
  have 
  become 
  so 
  familiar. 
  

  

  Measured 
  in 
  terms 
  of 
  the 
  length 
  of 
  a 
  wave, 
  electric 
  waves 
  extend 
  

   from 
  many 
  miles 
  in 
  length 
  down 
  through 
  the 
  radio 
  waves 
  of 
  say 
  300 
  

   meters, 
  to 
  the 
  very 
  short 
  waves 
  resulting 
  from 
  tiny 
  sparks, 
  which 
  may 
  

   be 
  no 
  more 
  than 
  a 
  tenth 
  of 
  a 
  millimeter 
  in 
  length. 
  These 
  rays 
  over- 
  

   lap 
  in 
  wave 
  length 
  the 
  longest 
  heat 
  waves 
  radiated 
  by 
  hot 
  bodies, 
  and 
  

   may 
  be 
  detected 
  and 
  measured 
  by 
  the 
  same 
  instruments. 
  A 
  familiar 
  

   source 
  of 
  such 
  heat 
  rays 
  is 
  the 
  reflector 
  type 
  of 
  electric 
  heater, 
  the 
  

   kind 
  that 
  warms 
  one 
  side 
  of 
  us 
  in 
  a 
  chilly 
  room. 
  The 
  greater 
  part 
  of 
  

   these 
  heat 
  rays 
  are 
  intermediate 
  in 
  wave 
  length 
  between 
  the 
  shortest 
  

   electric 
  waves 
  and 
  visible 
  light. 
  Such 
  a 
  heater, 
  however, 
  glows 
  a 
  dull 
  

  

  red, 
  showing 
  that 
  its 
  rays 
  extend 
  into 
  the 
  visible 
  region. 
  

  

  215 
  

  

  