Physics 123

1
Scattering of light

• Physics 123

2
Test 1

• March 8- in lecture class (1230-145pm)
• Equation sheet will be provided with the test
• Review it in advance available on the web
• HW6 is based on Chapters 33-37 to help you
  prepare for the test
• Lecture 14 review, send requests (e.g.
  homework problems)
• HW7 light and conceptual, but more work after
  spring break

3
Electron and EM wave

• Prediction from classical EM
• Electric field exerts a force on an electron ?
  acceleration ?electron radiates EM wave
• Oscillating electric field (frequency f) on
  electron ?forced oscillator with the same
  frequency ? radiation at the same frequency f
• An electron will scatter EM wave frequency f,
  wavelength l in all directions without changing
  its frequency and wavelength.
• Rayleigh scattering
• Blue-violet light gets scattered more than
  red-orange ? than is why the sky is blue

E
-
4
Photons particles of light

• Photon particle of light quantum of light
  gamma(g)-quant
• Each photon carries energy proportional to the
  frequency of the EM wave
• Eghfhc/l
• Photon has zero mass m00, it is always (and in
  any inertial frame of reference) moving at the
  speed of light vc
• Photons momentum can be calculated from
  relativistic formula
• pgh/l

5
Collisions Compton effect

• EM wave collection of photons
• Light particle photon scatters of electron at
  rest
• EgEgKEe
• Electron gets some energy ? photon looses energy
• Eg goes down ? f goes down ? wave length l goes
  up
• lgtl
• Expect maximum change (longest l) for back
  scattering (fp)

KEe
Eg
Eg
6
Compton effect

• Calculate wavelength of scattered photon as a
  function of angle with respect to its initial
  direction f
• Energy and momentum is conserved

Ee
Eg
Eg
Energy-momentum conservation
Dispersion equations Relate energy, momentum
and mass
7
Compton effect
Ee
Eg
Eg
8
Compton effect
Ee
Eg
Eg
9
Compton effect
Ee
Always positive change
Eg
No change no interaction
Max change Back scattering
Eg
Compton wavelength
10
Pair production

• A piece of antimatter Positron antipartner of
  electron same mass, opposite charge
• When matter and antimatter meet (positron and
  electron collide) they annihilate produce light
• It can go in the opposite direction as well
• A photon can create matter, such as the
  production of a positron and electron pair.
• Minimum energy of the photon for pair production
  from energy conservation
• Eg2mec22 0.5MeV1MeV

E-
Eg
e after collision
E
11
Light interaction with matter

• Rayleigh scattering
• Photoelectric effect photon is absorbed and an
  electron is knocked out
• Excitation absorbed by atoms that go to an
  excited state. Atom goes to ground state and
  emits a photon.
• Compton scattering wavelength is increased
• Electron-positron pair production

1-100 eV 500-10nm 1-10 eV 500nm 1-20 eV
500-100nm 0.5MeV
2.43pm gt1MeV
lt1.2pm
12
Light interaction with matter