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The Photo Electric Effect. Discovery, implications, and current technology. Presentation by Ryan Smith. Discovery: Heinrich Hertz and Phillip Lenard. Hertz clarified Maxwell ’ s electromagnetic theory of light: Proved that electricity can be transmitted in electromagnetic waves.

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The Photo Electric Effect

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The photo electric effect

The Photo Electric Effect

Discovery, implications, and current technology

Presentation by Ryan Smith

Discovery heinrich hertz and phillip lenard

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.

Back in 1887…

The photo electric effect

The Spark Gap Generator

  • First observed the effect while working with a spark-gap generator ~ accidentally, of course

  • Illuminated his device with ultraviolet light:

    • This changed the voltage at which sparks appeared between his electrodes!

Hertz s spark gap generator

Hertz’s Spark Gap Generator:

Lenard goes further

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

Lenard’s Photoelectric Apparatus:

The experiment

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

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

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.

This was published in his famous 1905 paper:

“On a Heuristic Point of View About the Creation and Conversion of Light”

Einstein s relations

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

…Therefore light energy comes in multiples of hv

Graph of ke max vs frequency

Graph of KEmax vs. frequency

Quantum leap for quantum mechanics

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.”

*This work won Einstein his Nobel Prize in 1922.*

The photo electric effect

Quantum Implications

Electrons must exist only at specific energy levels within an atom 

Work function ionization energy



Work Function ≈ Ionization Energy

  • Φ represents how hard it is to remove an electron…

  • Electron volts (eV)

  • Varies slightly

Emergent applications

Emergent Applications…

  • Response is linear with light intensity

  • Extremely short response time

  • For example, night vision devices:

The photo electric effect

At Nearly the Same Time,

Another Discovery is under way….

The photo electric effect

The PhotoVoltaic Effect:

  • Same basic principle as the photoelectric effect


  • In 1839, Alexandre Edmond Becquerel

  • In 1873, Willoughby Smith

  • In 1876, William Grylls Adams (with his student R. E. Day)

  • In 1883, the first “real” solar cell was built by Charles Fritts, forming p-n junctions by coating selenium with a thin gold layer.

The photo electric effect

P- and N-type Materials

  • N-Type: Requires doping a material with atoms of similar size, but having more valence electrons. ex/ Si:As

The photo electric effect

P- and N-type Materials

  • P-Type: Requires doping a material with atoms of similar size, but having fewer valence electrons. ex/ Si:Ga

The photo electric effect

Donor and Acceptor Bands

  • Dopants add quantum energy levels

  • Translate into bands in the solid semiconductor.

  • Formation of majority charge carriers on each side:



e- 

e- 

*extra positive “holes”

from electron vacancies

*extra negative electrons

The photo electric effect

Solar (PV) Cells:

  • Each material by itself is electrically neutral, however…

  • Joining P- and N-Type materials together creates an electric field at the junction between them ~

An equilibrium is reached where a net charge concentration exists on each side of the junction.

The photo electric effect

Solar (PV) Cells:

  • A photon is absorbed by the material near the P-N junction, creating an electron/hole pair:

The photo electric effect

The Electric Field Drives Current

  • Minority charge carriers are attracted to the junction

  • Majority charge carriers are repelled

The photo electric effect

Efficiency – the “Band Gap”

  • Only the right frequencies of light let an electron cross the junction, or “band gap”.

The big picture

The Big Picture:

The photo electric effect

Hopes for the Future

  • Multi-junction solar cells

  • improve efficiency.

  • Thin-film P-N junction

  • solar cells reduce material

  • use and cost.

  • Bring the current price per watt down



Austin, Geoff. Jan 2005. Photo Electric Effect. Retrieved 10-23-05.

Einstein, Albert. (1905). “On a Heuristic Viewpoint Concerning the Production and Transformation of Light.” Annalen der Physik, Vol 17, 132.

Elert, Glenn. Photoelectric Effect. Retrieved 10-28-05.

Hamakawa, Yoshihiro. (2004). Thin-Film Solar Cells: Next generation photovoltaics and its application. New York: Springer.

Lenardic, Denis. A Walk Through Time. Retrieved 11-12-05.

U.S. DOE Photovoltaics Program. (2005). Photovoltaics Timeline. Retrieved 10-27-05.

n.a. n.d. Philipp Lenard – Biography. Retrieved 10-23-05.

n.a. n.d. The Photo Electric Effect. Retrieved 10-06-05.

n.a. n.d. The Electric Field In Action. Retrieved 11-12-05.

n.a. n.d. Timeline of Solar Cells. Retrieved 10-27-05.

Robertson, E F. O’Conner, J J. A history of Quantum Mechanics. Retrieved 10-25-05.

Smith, Willoughby. (1873). "Effect of Light on Selenium during the passage of an Electric Current".Nature, Vol ? 303.

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