The Photoelectric Effect

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The Photoelectric Effect
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Is light a wave?
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Or is it a particle?
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The photoelectric effect is best explained by
treating light as a
Particle
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I was only 26 in 1905!!

• Albert Einstein explained the photo- electric
  effect in 1905.
• He won the Nobel Prize for his work in 1921.
• He also published his special theory of
  relativity in the same year.

1879-1955
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The Photoelectric effect occurs when light is
shown on a metal. The light ejects electrons
from the metal producing an electric current..
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METAL
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According to classical physics, high intensity
light would eject electrons with greater kinetic
energy. Experimental results contradicted this
theory.
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Experiments showed that the kinetic energy of the
electrons depended on the frequency of the light.

• Low frequency light, no matter how intense could
  not eject a single electron.
• High frequency light with a very small intensity
  was able to eject electrons wit a large kinetic
  energy

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Experiments were done to find the relationship
between the kinetic energy of the ejected
electrons and the frequency of the incident
light. They resulted in the following graph
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Slope h
Fo
max kinetic energy
frequency
y intercept ?
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Written Relationship

• The kinetic energy of the ejected electron is
  proportional to the frequency of the incident
  light.

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Slope h
Fo
max kinetic energy
frequency
y intercept ?
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Mathematical Model
Kmax hf - ?
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Light is a particle, called a photon. Each
particle has a energy that is directly
proportional to the frequency of the light.
E hf
E Energy h Plancks Constant (6.63x10-34
Js) f frequency of the light
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? represents the work function of the metal. It
is the minimum energy needed to eject an electron
from the metal.
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PRACTICAL APPLICATIONS

• The Photoelectric Cell
• video cameras
• electric eyes
• burglar alarms
• night lights
• solar power
• video games

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Example The work function for a particular metal
is 3.2x10-19J. If the metal is illuminated with
light of wavelength 550nm. What is the maximum
speed of the emitted electrons?
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• The maximum speed is
• 3.0x105m/s

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Sample Quiz

• The photoelectric effect is best explained by
  treating light as a ___________?
• Who won the Nobel Prize for the explaining the
  photoelectric effect?
• List two practical applications of the photo
  electric effect.