the photoelectric effect is best explained by treating light as a particle
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The photoelectric effect is best
explained by treating
light as a
Particle
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• 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.
I was only 26 in 1905!!
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
PHOTONS
ee
ee ELECTRONS
e
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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 with 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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frequency
y intercept
Slope hFo
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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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frequency
y intercept
Slope hFo
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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 = hfE = Energy
h = Planck’s 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.