Compton Effect

1
Compton Effect

• Einstein said light consists of quanta of energy
  (1905)
• He did NOT treat momentum until later (1906)
• He said momentum E/c hf/c
• Debye and Compton treated photon-particle
  collisions much later (1923)

2
X-Rays

• Discovered in 1895 by Roentgen
• High speed electrons onto a metal target and get
  a new highly penetrating radiation
• Medical X-Rays came within months
• EM waves with very short wavelengths (very high
  frequency)
• Wavelengths of about 10-10 meters same spacing
  as atoms in crystals

3
Spectrum of X-Rays

• Broad continuous spectrum
• Characteristic lines from different elements
• Bremsstrahlung (braking radiation)
• eVhfhc/min
• minhc/eV where V is tube voltage
• Already evidence of particle nature of x-rays

4
X-Ray Scatting from Crystals

• Atoms in successive planes scatter constructively
  if path difference is a whole number of
  wavelengths n2dsin n123.

A
C


B
5
Compton Effect

• Considered X-rays scattering from free electrons
• Classically should get re-radiation at equal or
  lower frequencies
• Classically re-radiated frequency depends on
  intensity and exposure time
• Experimentally frequency shift is independent of
  time and intensity

6
Compton Effect

• Wavelength shift depends only on scattering angle
• Consider a collision between a photon and a free
  electron
• Photon energy is hf and momentum is hf/c
• Do a relatavistic calculation

7
Compton Effect
Ee. pe
Photon E p


EltEp
8
Compton Effect
9
Compton Effect

• Eliminate and solve for pe

10
Compton Effect

• So shift only depends on angle and gives a
  result that agrees with experiment
• Problem Solved!!!!