Activity 20:  Photoelectric Effect

This activity is more a demo than a hands-on activity, but you should record the data taken in your class and answer the analysis questions following the data table.

Equipment needed:  A spectral lamp, lens/diffraction grating apparatus, photoelectric head, voltmeter, and banana plug wires.

Background

In today's experiment, light shines on electrons in a metal.  The electrons in the metal must absorb the binding energy, or work function f, of the metal to be released.  Any energy above f goes into kinetic energy of the electrons.  Thus the kinetic energy of electrons is
Kmax = Eabsorbed - f
The max subscript indicates that only those electrons that do not lose energy in collisions on their way out of the metal will have this value of kinetic energy; others will have less kinetic energy.

The energy of waves is proportional to its intensity, and is spread over the entire area of the wave.  Thus wave theory predicts that Eabsorbed, and therefore Kmax, should be linear in (directly proportional to) intensity and have no dependence upon frequency.
 
The equipment for today's activity uses a mercury lamp and a diffraction grating to produce 5 different distinct frequencies of light.  The diffraction grating separates the mercury spectrum into its component wavelengths.  We will let the different spectra lines shine one at a time on a piece of metal enclosed in the photoelectric head.  As electrons leave the metal (called the cathode since it is a source of electrons), they are collected on another surface (called the anode).  This sets up a potential difference V between the now-positively charged cathode and the now-negatively charged anode, just like the potential difference across a capacitor.  Remembering the definition of potential difference, we can see that an electron moving from the cathode to the anode will gain potential energy of DU = eV.  In the process, the electron will lose an equivalent amount of kinetic energy:  DK = -eV.  Once the potential difference between the plates is so high that electrons with Kmax cannot make it across, the capacitor stops charging, and this maximum potential difference is called the stopping potential, Vs:
Kmax = eVs = Eabsorbed - f

 

Taking Measurements
 

Color
l (nm)
f (Hz)
V (V)
K (J)
  yellow 
 578.1 
  5.19E+14    
green
546.1
 5.49E+14    
blue
435.8
 6.88E+14    
violet
404.7
 7.41E+14    
u.v.
365.0
 8.22E+14    
Intensity
V (V)
K (J)
  100% 
    
80%
    
60%
    
40%
    
20%
    
1. Copy the tables above on your page, and fill them in as the instructor takes the measurements.  You may choose to follow along, entering the data in the spreadsheet at Act20xl.xls.
2. Record the slopes and intercepts of the linear fit from each graph.
3. Does stopping potential (and thus maximum kinetic energy) appear to depend upon frequency of light or upon intensity of light?  Justify your answer.
4. Compare the slope of the K vs. f graph to to the accepted value of Planck's constant, h=6.63x10-34 Js.
5. How soon after you release the Zero buttons do electrons start flowing?  Is this consistent with waves or particles?
6. What part(s) of the experiment demonstrated the wave nature of light?  the particle nature? 

You will be asked to complete an evaluation of today's activity and lecture before the end of class.  This evaluation counts as a free 5% of each activity grade.  It will generally be done on WebCT in the last 5-10 minutes of class, but time constraints may lead to the occasional evaluation done on paper.