The photoelectric effect

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• The photoelectric effect
• Contents
• Einsteins proposed experiment
• Solving photoelectric problems
• Example 1 Example 2
• Whiteboard
• Photon vs wave theory

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Light
Waves Photons
Wavelength Changes
Energy per photon changes E hf X-Rays, UV, Gamma
Color
Amplitude Changes
of photons changes many bright few dim CCD
Devices, High speed film
Brightness
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• Einstein
• Proposes Photon theory
• Experiment

Light ejects electrons Ammeter detects

• Some Potential V stops all electrons
• This is called the Stopping potential
• (Pretty tough, huh?)

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Consider an ejected electron hurtling toward the
negative plate
Remember V W/q, so W Vq Definition of
electron volt Ekin turns to potential energy If
5.12 V is the stopping potential, then Ekin
5.12 eV
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Einsteins Photon theory predicts Photon energy
Work function Kinetic energy of electron
hf ?
Ekmax hf
hfo eVs
? - Work function (Depends on material) fo -
Lowest frequency that ejects e - Electron
charge Vs - The uh stopping potential
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Work Function
Metal
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Photon energy Work function Kinetic energy of
electron hf ?
Ekmax hf
hfo
eVs
? - Work function fo - Lowest frequency that
ejects e - Electron charge Vs - The uh stopping
potential
e 1.602 x 10-19 C V W/q E hf hc/?
Example 1 A certain metal has a work function of
3.25 eV. When light of an unknown wavelength
strikes it, the electrons have a stopping
potential of 7.35 V. What is the wavelength of
the light?
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Photon energy Work function Kinetic energy of
electron hf ?
Ekmax hf
hfo
eVs
? - Work function fo - Lowest frequency that
ejects e - Electron charge Vs - The uh stopping
potential
e 1.602 x 10-19 C V W/q E hf hc/?
Example 2 70.9 nm light strikes a metal with a
work function of 5.10 eV. What is the maximum
kinetic energy of the ejected photons in eV?
What is the stopping potential?
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Whiteboards Photoelectric effect 1 2 3 4
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Photons of a certain energy strike a metal with a
work function of 2.15 eV. The ejected electrons
have a kinetic energy of 3.85 eV. (A stopping
potential of 3.85 V) What is the energy of the
incoming photons in eV?
Photon energy Work function Kinetic energy of
electron Photon energy 2.15 eV
3.85 eV 6.00 eV
W
6.00 eV
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Another metal has a work function of 3.46 eV.
What is the wavelength of light that ejects
electrons with a stopping potential of 5.00 V? (2)
E hf hc/?, Photon energy Work function
Kinetic energy of electron Photon energy 3.46
eV 5.00 eV 8.46 eV E (8.46
eV)(1.602 x 10-19 J/eV) 1.3553 x 10-18 J E
hf hc/?, ? hc/E (6.626 x 10-34 Js)(3.00 x
108 m/s)/(1.3553 x 10-18 J) ? 1.4667x 10-07 m
147 nm
W
147 nm
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112 nm light strikes a metal with a work function
of 4.41 eV. What is the stopping potential of
the ejected electrons? (2)
E hf hc/?, 1 eV 1.602 x 10-19 J Photon
energy Work function Kinetic energy of
electron E hf hc/? (6.626 x 10-34 Js)(3.00
x 108 m/s)/(112 x 10-9 m) E 1.7748 x 10-18 J E
(1.7748 x 10-18 J)/(1.602 x 10-19 J/eV)
11.079 eV Photon energy Work function
Kinetic energy of electron 11.079 eV 4.41
eV eVs 11.079 eV - 4.41 eV 6.6688
eV eVs Vs 6.67 V
W
6.67 V
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256 nm light strikes a metal and the ejected
electrons have a stopping potential of 1.15 V.
What is the work function of the metal in eV? (2)
E hf hc/?, 1 eV 1.602 x 10-19 J Photon
energy Work function Kinetic energy of
electron E hf hc/? (6.626 x 10-34 Js)(3.00
x 108 m/s)/(256 x 10-9 m) E 7.7648 x 10-19 J E
(7.7648 x 10-19 J)/(1.602 x 10-19 J/eV) 4.847
eV Photon energy Work function Kinetic
energy of electron 4.847 eV Work
function 1.15 eV 11.079 eV - 1.15 eV 3.70 eV
W
3.70 eV
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Einsteins Photon theory predicts Photon energy
work function Kinetic energy of electron hf
? Ekmax Ekmax hf - ?

• Photon Theory predicts
• Ekmax rises with frequency
• Intensity of light ejects more

• Wave Theory predicts
• Ekmax rises with Amplitude (Intensity)
• Frequency should not matter
• Survey says

Millikan does the experiment 1915 conclusion 1930
conclusion
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