The photo electric effect
This presentation is the property of its rightful owner.
Sponsored Links
1 / 27

The Photo Electric Effect PowerPoint PPT Presentation


  • 103 Views
  • Uploaded on
  • Presentation posted in: General

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.

Download Presentation

The Photo Electric Effect

An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Presentation Transcript


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

Φ

Φ

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

  • HISTORY

  • 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:

N-Type

P-Type

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


References

References:

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

http://www.eequalsmcsquared.auckland.ac.nz/sites/emc2/tl/pee/overview.cfm

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.

http://hypertextbook.com/physics/modern/photoelectric/

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. http://www.pvresources.com/en/history.php

U.S. DOE Photovoltaics Program. (2005). Photovoltaics Timeline. Retrieved 10-27-05. http://inventors.about.com/library/inventors/blsolar2.html

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

http://nobelprize.org/physics/laureates/1905/lenard-bio.html

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

http://www.lancs.ac.uk/ug/jacksom2/

n.a. n.d. The Electric Field In Action. Retrieved 11-12-05. http://www.sandia.gov/pv/docs/PVFEffElectric_Field.htm

n.a. n.d. Timeline of Solar Cells. Retrieved 10-27-05. http://www.nationmaster.com/encyclopedia/Timeline-of-solar-cells

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

http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/The_Quantum_age_begins.html

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

Available URL: http://histv2.free.fr/selenium/smith.htm


  • Login