Photoelectric Effect

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Photoelectric Effect
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What is Light?
ELECTROMAGNETIC THEORY (1862)
Waves of electric and magnetic energy
Evidence interference, polarization,
diffraction

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Radiation causes electrons to be ejected from
metal plate
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At stopping voltage V0, no electrons have enough
KE to reach other plate.
KEmax
eV0
qV
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Experiment vary intensity of radiation vary
frequency of radiation
Predictions based on EM theory
1. increased intensity
more electrons greater KE
2. increased frequency
no effect
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Experimental Results
1. more electrons no change in KE
2. below a certain cut-off frequency, no
electrons emitted, no matter how intense the
light. Above the minimum frequency,
KEmax increases as freq. increases
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Each photon has energy E hf (E hc/?)
Intensity (power/area) corresponds to number of
photons.
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Electron can interact with only one photon at a
time
KEmax hf W0
E hf
photon
e-
W0 work function (minimum energy to
release electron)
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W0 hf0 hc/?0
f0 (threshold frequency) for potassium
4.83 x 1014 Hz
?0 (threshold wavelength)
6.21 x 10-7 m 621 nm
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If green light (2.25 eV)
KEmax vmax V0
0.25 eV
2.96 x 105 m/s
0.25 V