Ain Shams U. Faculty of Engineering Mathematics and Engineering Physics Department Lecture 2 : Photoelectric Effect

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Ain Shams U.Faculty of EngineeringMathematics
and Engineering Physics DepartmentLecture 2
Photoelectric Effect

• ?. ???? ???????
• ?????? 2004

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Contents

• Websites
• Historical background
• Photoelectric phenomena
• Classical physics prediction
• Modern physics explanation

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Websites

• http//lectureonline.cl.msu.edu/mmp/kap28/PhotoEf
  fect/photo.htm
• http//theory.uwinnipeg.ca/physics/quant/node3.htm
  l
• http//www.colorado.edu/physics/2000/quantumzone/p
  hotoelectric.html

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Historical background

• 1888, Hallwachs found that when a Zinc plat is
  illuminated with ultraviolet light, it becomes
  positively charged.
• 1899, Thomson showed that electrons were expelled
  from the plate.
• 1902, Lenard experiment.
• 1905, Einstein explanation.

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Photoelectric effect

• If the incident wave frequency (f) is smaller
  than a certain value (fo), there is no current
  flows.
• If fgt fo, the current flows instantaneously.

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Photoelectric effect

• With C forward biased, the electrons are
  accelerated faster resulting in an increasing
  current till a saturating value.
• The higher the incident wave intensity, the
  higher the saturation current.
• With C reverse biased, electrons are repelled
  from reaching C, and thus the current decreases.
• At a certain reverse potential -Vo the current
  becomes zero independent on the incident wave
  intensity.

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Classical physics prediction

• Electrons can be emitted regardless of the
  incident frequency, though it will take longer
  time for smaller incident wave amplitude.
• There should be a time delay between the wave
  illumination and the emission of electrons.
• The higher the wave intensity, the higher
  electron energy, and thus the higher the stopping
  voltage.

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Modern physics explanation

• The electromagnetic wave consists of many lumped
  energy particles called photons.
• The energy of each individual photon is given by

Jole
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Modern physics explanation

• If N is the total number of photons incident
  during time interval T, then the total incident
  optical energy is
• The incident energy per second (power) is given
  by
• nN/T is the number of incident photons per
  second.

Jole
Watt J/Sec.
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Modern physics explanation

• Interaction (absorption / emission) between the
  electromagnetic wave and matter occurs through
  annihilation (?????? ???) / creation (???) of a
  quantized (???) energy (photon).
• In the photoelectric effect, each single absorbed
  photon gives its total energy (hf) to one single
  electron.
• This energy is used by the electron to
• Overcome the attraction force of the material.
• Gain kinetic energy when freed from the material.

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Modern physics explanation

• Work function (?) It is the minimum required
  energy required by an electron to be free from
  the attraction force of the metal ions.
• Some of the electrons may need more energy than
  the work function to be freed.

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Modern physics explanation
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Modern physics explanation
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Modern physics explanation

• The electrons that need only the work function to
  be freed, will have the greatest kinetic energy
  outside the metal.
• The electrons requiring higher energy to be
  freed, will have lower kinetic energy.

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Modern physics explanation

• Thus, there is a minimum required photon energy
  (hfo) to overcome the work function of the
  material.
• If the incident photon energy is less than the
  work function, the electron will not be freed
  from the surface, and no photoelectric effect
  will be observed.

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Modern physics explanation

• The most energetic electrons are stopped by the
  reverse biased stopping potential -Vo.

Kinetic energy of photoelectron potential
energy created by the reverse biased connection
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Modern physics explanation

• The stopping potential doesnt depend on the
  incident light intensity.
• The stopping potential depends on the incident
  frequency.