﻿220 
  ANNUAL 
  REPORT 
  SMITHSONIAN 
  INSTITUTION, 
  19 
  2 
  9 
  

  

  drawn 
  we 
  have 
  the 
  single 
  vertical 
  line 
  D. 
  With 
  the 
  grating 
  in 
  place 
  

   we 
  see 
  a 
  bright 
  central 
  reflected 
  image 
  O 
  with 
  companions 
  on 
  either 
  

   side. 
  Thus 
  X 
  rays 
  can 
  also 
  be 
  diffracted, 
  and 
  must 
  therefore, 
  like 
  

   light, 
  consist 
  of 
  waves. 
  

  

  LIGHT 
  CONSISTS 
  OF 
  PARTICLES 
  

  

  For 
  a 
  hundred 
  years 
  no 
  one 
  had 
  seriously 
  questioned 
  the 
  truth 
  of 
  the 
  

   wave 
  theory. 
  In 
  1900, 
  however, 
  Planck 
  published 
  the 
  results 
  of 
  a 
  

   long 
  study 
  of 
  the 
  problem 
  of 
  the 
  radiation 
  of 
  heat 
  and 
  light 
  from 
  a 
  hot 
  

   body. 
  This 
  difficult 
  theoretical 
  study, 
  which 
  has 
  stood 
  the 
  test 
  of 
  

   time, 
  showed 
  that 
  if 
  a 
  body 
  when 
  heated 
  is 
  to 
  become 
  first 
  red 
  hot, 
  

   then 
  yellow, 
  and 
  then 
  white, 
  the 
  oscillators 
  in 
  it 
  which 
  are 
  giving 
  out 
  

   the 
  radiation 
  must 
  not 
  radiate 
  continuously 
  as 
  the 
  electromagnetic 
  

   theory 
  would 
  demand. 
  They 
  must 
  rather 
  radiate 
  suddenly 
  little 
  

   portions 
  of 
  energy. 
  The 
  amount 
  of 
  energy 
  in 
  each 
  portion 
  must 
  

   further, 
  according 
  to 
  Planck, 
  be 
  proportional 
  to 
  the 
  frequency. 
  

   This 
  is 
  the 
  origin 
  of 
  the 
  celebrated 
  ''quantum" 
  theory. 
  

  

  On 
  account 
  of 
  the 
  difficult 
  character 
  of 
  the 
  reasoning 
  involved 
  in 
  

   Planck's 
  argument, 
  his 
  conclusions 
  carried 
  weight 
  only 
  among 
  those 
  

   who 
  were 
  especially 
  interested 
  in 
  theoretical 
  physics. 
  Among 
  these 
  

   was 
  Einstein, 
  who 
  called 
  attention 
  to 
  the 
  fact 
  that 
  Planck's 
  con- 
  

   clusions 
  would 
  fit 
  exactly 
  with 
  the 
  view 
  that 
  the 
  radiation 
  was 
  not 
  

   emitted 
  in 
  waves 
  at 
  all, 
  but 
  as 
  little 
  particles, 
  each 
  possessing 
  a 
  por- 
  

   tion 
  of 
  energy 
  proportional 
  to 
  the 
  frequency 
  of 
  the 
  oscillator, 
  as 
  

   Planck 
  had 
  assumed. 
  

  

  Einstein 
  and 
  the 
  photoelectric 
  efect. 
  — 
  An 
  opportunity 
  to 
  apply 
  this 
  

   idea 
  was 
  afforded 
  by 
  the 
  photoelectric 
  effect. 
  It 
  is 
  found 
  that 
  when 
  

   light, 
  as 
  from 
  an 
  arc, 
  falls 
  upon 
  certain 
  metals, 
  such 
  as 
  zinc 
  or 
  sodium, 
  

   a 
  current 
  of 
  negative 
  electricity 
  in 
  the 
  form 
  of 
  electrons 
  escapes 
  from 
  

   the 
  metallic 
  surface. 
  This 
  photoelectric 
  eft'ect 
  is 
  especially 
  promi- 
  

   nent 
  with 
  X 
  rays, 
  for 
  these 
  rays 
  eject 
  electrons 
  from 
  all 
  sorts 
  of 
  sub- 
  

   stances. 
  In 
  Plate 
  3, 
  Figure 
  1, 
  is 
  shown 
  one 
  of 
  C. 
  T. 
  R. 
  Wilson's 
  

   photographs 
  of 
  the 
  trails 
  left 
  by 
  electrons 
  ejected 
  by 
  X 
  rays 
  passing 
  

   through 
  air 
  and 
  a 
  sheet 
  of 
  copper. 
  These 
  electrons, 
  shot 
  out 
  of 
  the 
  

   air 
  and 
  the 
  metal 
  by 
  the 
  action 
  of 
  the 
  X 
  rays, 
  are 
  the 
  X-ray 
  photo- 
  

   electrons. 
  

  

  The 
  most 
  remarkable 
  property 
  of 
  these 
  photoelectrons 
  is 
  the 
  speed 
  

   at 
  which 
  they 
  move. 
  We 
  have 
  seen, 
  as 
  in 
  Figure 
  4, 
  that 
  X 
  rays 
  are 
  

   the 
  waves 
  produced 
  when 
  the 
  cathode 
  electrons 
  bombard 
  a 
  metal 
  

   target 
  inside 
  the 
  X-ray 
  tube. 
  Let 
  us 
  suppose 
  that 
  a 
  cathode 
  electron 
  

   strikes 
  the 
  target 
  at 
  a 
  speed 
  of 
  a 
  hundred 
  thousand 
  miles 
  a 
  second 
  

   (they 
  move 
  tremendously 
  fast). 
  The 
  resulting 
  X 
  ray, 
  after 
  passing 
  

   through 
  the 
  walls 
  of 
  the 
  X-ray 
  tube 
  and 
  perhaps 
  a 
  block 
  of 
  wood, 
  may 
  

   eject 
  a 
  photoelectron 
  from 
  a 
  metal 
  plate 
  placed 
  on 
  the 
  far 
  side. 
  The 
  

   speed 
  of 
  this 
  photoelectron 
  is 
  then 
  found 
  to 
  be 
  almost 
  as 
  great 
  as 
  

   that 
  of 
  the 
  original 
  cathode 
  electron. 
  

  

  