The Photoelectric Effect

1
The Photoelectric Effect
Shreyes Seshasai MIT Junior Lab, Fall 2006
2
Talk Outline

• Introduction
• Theory
• Experiment Set-up and Data Collection
• Results and Error Analysis
• Conclusion

3
Introduction

• Photoelectric Effect demonstrates the
  wave-particle duality of light
• Observed by Heinrich Hertz in 1887, when he
  noticed light from sparks would induce a current
• Einstein proposed the idea of light quanta in
  1905, with light consisting of discrete quanta of
  energy h?

4
The Theory behind the Effect

• Light consists of quanta of energy, E h?
• Metals have a specific work function, ?,
  corresponding to the minimum amount of energy
  required to excite an electron
• If the energy of incident photons is greater than
  ?, electrons will be emitted with the difference
  in energy
• This can be summarized by
• K h? - ?
• There is a one-to-one interaction between photons
  and electrons, so the intensity of the light
  doesnt affect energy of the emitted electron

5
Setting up the Experiment

• Mercury Lamp emits a
• specific spectrum
• Filters are
• 360.0 2.0 nm
• 404.7 2.0 nm
• 435.8 2.0 nm
• 546.1 2.0 nm
• 577.0 2.0 nm

Source The Photoelectric Effect, MIT 8.13 Lab
Guide. http//web.mit.edu/8.13/www/JLExperiments/J
LExp005.pdf
6
Data Collection
7
Data Collection (cont)
Note currents normalized to Dark Purple filter
8
Data Collection (cont)
Note currents normalized to Dark Purple filter
9
Analyzing the Data

• Stoppage voltage determined in two ways
• (a) by finding a linear approximation for the
  I vs V curve and determining when I 0
• (b) by estimating where the current begins to
  rise

10
Calculating h and ?

• Ee h? ?
• Ee energy of the electron
• h Plancks constant
• ? the frequency of the incident photon
• ? the work function of the metal

11
Results
12
Calculations

• Solving for Plancks constant, h and the work
  function ?
• (a) linear fit V -0.9191 2.119310-15 (?)
• h 2.119310-15(1.610-19)
• 3.4 2 10-34 J-s
• ? 0.92 0.5 V
• Taking into account the contact potential
  difference between the electrodes, ?A - ?C,, ?
  .92 3.4 4.3 0.5 V
• (b) linear fit V -1.3566 3.175510-15 (?)
• h 3.175510-15(1.610-19)
• 5.1 2 10-34 J-s
• ? 1.36 0.5 V
• Corrected value ? 1.36 3.4 4.8 0.5 V

13
Error Analysis

• Possible sources of error include
• Systematic error with the machinery/filters
• Induced current from external sources (very
  sensitive to wire movement)
• For the work function, gases in the photocell
  interact with the cathode surface

14
Conclusion

• Results for Planks constant are on the same
  order as the accepted value
• 3.410-34 J-s and 5.110-34 J-s
• (Accepted value h 6.62610-34 J-s)
• Results for the work function
• ? 4.3 0.5 V and 4.8 0.5 V
• (Published values are ?C 2.3, ?A 5.7)1

1. The Photoelectric Effect, MIT 8.13 Lab Guide.
http//web.mit.edu/8.13/www/JLExperiments/JLExp005
.pdf