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The Photo Electric Effect
Discovery, implications, and current technology
Discovery: Heinrich Hertz and Phillip Lenard
Hertz clarified Maxwell’s electromagnetic theory of light:
Proved that electricity can be transmitted in electromagnetic waves.
Established that light was a form of electromagnetic radiation.
First person to broadcast and receive these waves.
Hertz’s Spark Gap Generator:
Lenard Goes Further…
His assistant, Phillip Lenard, explored the effect further. He built his own apparatus called a “phototube” to determine the nature of the effect:
Lenard’s Photoelectric Apparatus:
The Experiment:
By varying the voltage on a negatively charged grid between the ejecting surface and the collector plate, Lenard was able to:
Determine that the particles had a negative charge.
Determine the kinetic energy of the ejected particles.
Lenard’s Findings:
Thus he theorized that this voltage must be equal to the maximum kinetic energy of the ejected particles, or:
KEmax = eVstopping
Perplexing Observations:
The intensity of light had no effect on energy
There was a threshold frequency for ejection
Classical physics failed to explain this,
Lenard won the Nobel Prize in Physics in 1905.
Einstein’s Interpretation
A new theory of light:
Electromagnetic waves carry discrete energy packets
The energy per packet depends on wavelength, explaining Lenard’s threshold frequency.
More intense light corresponds to more photons, not higher energy photons.
Einstein’s Relations:
Einstein predicted that a graph of the maximum kinetic energy versus frequency would be a straight line, given by the linear relation:
KE = hv - Φ
Graph of KEmax vs. frequency
Quantum leap for quantum mechanics
Wave-particle duality set the stage for 20th century quantum mechanics.
In 1924, Einstein wrote:
“…There are therefore now two theories of light, both indispensable, and - as one must admit today despite twenty years of tremendous effort on the part of theoretical physicists - without any logical connection.”
Work Function ≈ Ionization Energy
Φ represents how hard it is to remove an electron…
Electron volts (eV)
Varies slightly
Emergent Applications…
Response is linear with light intensity
Extremely short response time
For example, night vision devices: