1. Scattering of light
Physics 123
11/15/2018
Lecture XI

2. Test #1 March 8- in lecture class (12:30-1:45pm)
Equation sheet will be provided with the test
Review it in advance – available on the web
HW#6 is based on Chapters to help you prepare for the test
Lecture #14 – review, send requests (e.g. homework problems)
HW#7 – light and conceptual, but more work after spring break
11/15/2018
Lecture XI

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 -
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Lecture XI

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:
Eg=hf=hc/l
Photon has zero mass m0=0, it is always (and in any inertial frame of reference) moving at the speed of light v=c
Photon’s momentum can be calculated from relativistic formula
pg=h/l
11/15/2018
Lecture XI

5. Collisions: Compton effect
EM wave – collection of photons
Light particle – photon scatters of electron at rest:
Eg=Eg’+KEe
Electron gets some energy  photon looses energy
Eg goes down  f goes down  wave length l goes up:
l’>l
Expect maximum change (longest l) for back scattering (f=p)
KEe Eg
Eg’
11/15/2018
Lecture XI

6. Compton effect Ee Eg Eg’
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
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Lecture XI

7. Compton effect Ee Eg
Eg’
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8. Compton effect Ee Eg
Eg’
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9. Compton effect Ee Eg Eg’ Always positive change No change
~no interaction
Max change
Back scattering
Eg’
Compton wavelength
Note how small!
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Lecture XI

10. Pair production E- Eg E+
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:
Eg=2mec2=2 0.5MeV=1MeV E- Eg
e+ after
collision E+
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Lecture XI

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 nm
~1-10 eV ~500nm
1-20 eV nm
0.5MeV pm
>1MeV <1.2pm
11/15/2018
Lecture XI

12. Light interaction with matter
11/15/2018
Lecture XI