1. Photoelectric effect, photons
Physics 123
11/20/2018
Lecture X

2. Concepts Photoelectric effect Work function and stopping potential
ElectronVolt (eV)
Photons
Blackbody radiation
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Lecture X

3. Photoelectric effect Light shines on metal surface P
Electrons are emitted from P
Current flows between P and C
Measure kinetic energy of emitted electrons KEe by applying a stopping potential V0:
KEe=eV0
No surprises so far, because light as a wave carries energy
11/20/2018
Lecture X

4. Electronvolt
Energy that one electron gains when being accelerated over 1V potential difference is called electronvolt eV:
1eV=1.6x10-19C 1V= 1.6x10-19J
Yet another unit to measure energy,
Commonly used in atomic and particle physics.
11/20/2018
Lecture X

5. Photoelectric effect
Inside metal electrons are sitting in potential “wells”
Need supply some minimum energy W0 to get them out
W0 is called work function – different for different materials
Light provides this energy EL
EL=Ne(W0+KEe)
Ne- number of emitted electrons e- W0
light
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Lecture X

6. Photoelectric effect EL=Ne(W0+KEe) Here is a surprise:
Increase light intensity EL– expect
More electrons Ne - True
With higher kinetic energy KEe – False
Do not expect KE to depend on light wavelength l , but it does
More over if l>l0 – no electrons come out no matter how intensive the light source is!!!
11/20/2018
Lecture X

7. Photoelectric effect W0 Explanation of photoeffect A.Einstein
Postulate: light is transmitted in tiny particles (!!) – photons (g)
Each photon carries energy proportional to its frequency:
Eg=hf=hc/l
Planck’s constant h=6.626 x J.s
hc=1243 eV.nm
One electron absorbs only one photon
If Eg<W0 – no electrons are emitted:
Eg<W0 f<f0 l>l0 Eg Eg Eg e- W0
f0=W0/h
l0=hc/W0
11/20/2018
Lecture X

8. Photoelectric effect Eg=hf=hc/l
Photon energy (Eg) is spent to get electron out (W0) and on electron’s kinetic energy (KEe):
Eg= W0+ KEe
If stopping potential is applied electron’s kinetic energy KEe is converted into potential energy of the electron eV0:
Eg= W0+ eV0 Eg Eg Eg
KEe e- W0
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Lecture X

9. Photons – particles of light
Photon= particle of light = quantum of light = gamma(g)-quant
Intensity of electromagnetic wave – sum of energies carried by quanta of light – photons:
I=NEg
Each photon carries energy proportional to the frequency of the EM wave:
Eg=hf=hc/l
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Lecture X

10. Problem 38-15 I=0 when l>l0=570nm Work function W0-?
What stopping potential (V0) must be applied if light of l=400 nm is used?
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Lecture X

11. Photon absorption and emission
Photons can be absorbed by matter – converted into other forms of energy – e.g. kinetic, potential, thermal. At this point photon ceases to exist
Final energy=Initial energy+Eg
Photons can be emitted thus reducing the energy of the remaining system. Photon was NOT hiding inside matter waiting to be released, it is created by converting other forms of energy into light (EM-wave)
Final energy+Eg=Initial energy
Energy is conserved in either case
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Lecture X

12. More particle properties of light
Sun tan –
chemical reaction with threshold energy W0
visible light l= nm does not have enough energy to start this reaction
ultraviolet – l<400 nm - more energetic light – does have enough energy
Photographic film exposure
Why red light is safe? Eg Eg Eg
KEe e- W0
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Lecture X

13. Blackbody radiation
Another problem was elegantly solved by introducing “light particles”
Classical explanation of Blackbody (no reflection radiation diverged at low wavelength
Max Planck suggested replacing integral over frequencies with summing a series (photons!)
peak lpT=2.9x10-3 mK
Planck did not seek deep meaning behind this seemingly mathematical trick
Fundamental explanation was suggested by Einstein in 1905 – energy is quantized
11/20/2018
Lecture X