﻿WHAT 
  IS 
  LIGHT? 
  COMPTON 
  

  

  221 
  

  

  The 
  surprising 
  nature 
  of 
  this 
  phenomenon 
  may 
  be 
  illustrated 
  by- 
  

   considering 
  a 
  similar 
  event 
  with 
  water 
  waves. 
  Imagine 
  two 
  diving 
  

   boards 
  on 
  opposite 
  sides 
  of 
  a 
  wide 
  pond. 
  A 
  boy 
  dives 
  from 
  one 
  board 
  

   into 
  the 
  water 
  with 
  a 
  splash 
  which 
  sends 
  ripples 
  out 
  over 
  the 
  pool. 
  

   By 
  the 
  time 
  they 
  reach 
  the 
  second 
  boy, 
  who 
  is 
  swimming 
  in 
  the 
  water 
  

   beside 
  the 
  other 
  diving 
  board 
  some 
  distance 
  away, 
  these 
  ripples 
  are 
  

   much 
  too 
  small 
  to 
  notice. 
  We 
  should 
  be 
  greatly 
  surprised 
  if 
  these 
  

   insignificant 
  ripples 
  should 
  lift 
  the 
  second 
  swimmer 
  bodily 
  from 
  the 
  

   water 
  and 
  set 
  him 
  on 
  his 
  diving 
  board. 
  

  

  If, 
  however, 
  it 
  is 
  impossible 
  for 
  a 
  water 
  ripple 
  to 
  do 
  such 
  a 
  thing 
  it 
  

   is 
  just 
  as 
  impossible 
  for 
  an 
  ether 
  ripple, 
  sent 
  out 
  when 
  an 
  electron 
  dives 
  

  

  FiGUBE 
  4.- 
  

  

  -The 
  speed 
  of 
  the 
  photoelectrons 
  ejected 
  from 
  the 
  metal 
  plate 
  at 
  P 
  is 
  almost 
  as 
  great 
  as 
  the 
  

   speed 
  of 
  the 
  cathode 
  electrons 
  which 
  produce 
  the 
  X 
  rays 
  at 
  the 
  target 
  T 
  

  

  into 
  the 
  target 
  of 
  an 
  X-ray 
  tube, 
  to 
  jerk 
  an 
  electron 
  out 
  of 
  a 
  second 
  

   piece 
  of 
  metal 
  with 
  a 
  speed 
  equal 
  to 
  that 
  of 
  the 
  first 
  electron. 
  

  

  It 
  was 
  considerations 
  of 
  this 
  kind 
  which 
  showed 
  to 
  Einstein 
  the 
  

   futility 
  of 
  trying 
  to 
  account 
  for 
  the 
  photoelectric 
  effect 
  on 
  the 
  basis 
  of 
  

   waves. 
  He 
  saw, 
  however, 
  that 
  this 
  effect 
  might 
  be 
  explained 
  if 
  light 
  

   and 
  X 
  rays 
  consist 
  of 
  particles. 
  These 
  particles 
  are 
  now 
  commonly 
  

   called 
  " 
  photons." 
  The 
  picture 
  of 
  the 
  X-ray 
  experiment 
  on 
  this 
  

   view 
  would 
  be 
  that 
  when 
  the 
  electron 
  strikes 
  the 
  target 
  of 
  an 
  X-ray 
  

   tube, 
  its 
  energy 
  of 
  motion 
  is 
  transformed 
  into 
  a 
  photon, 
  that 
  is, 
  a 
  

   particle 
  of 
  X 
  rays 
  which 
  goes 
  with 
  the 
  speed 
  of 
  light 
  to 
  the 
  second 
  piece 
  

  

  