Chapter 15

Photoelectric Effect Lab

Open the Photoelectric simulator and answer each of the following exercises. As you answer each question, keep in mind the photon theory of light and the fact that electrons are negatively charged particles.
The exercise assumes that the simulation has Beam Control at 0%, wavelength at 400nm, Voltage=0V, and Target=Sodium.
  1. Increase the light intensity - topmost slider labelled "Beam Control": What is the effect, and why?
  2. Leaving the intensity at maximum, record the wavelength and current.
    Change the wavelength to different values and record the corresponding current, then Make a graph of current versus wavelength.
    How does this graph change if the intensity is decreased to about halfway (run the numbers)?
  3. Adjust the battery to a positive value; what is the effect, and why?
    Adjust the battery to a negative value; what is the effect, and why?
    Do not proceed until you have convinced the instructor you know what's happening.
    Hint: the red + signs and blue - signs on the electric plates represent the actual effect the battery has on those plates.
    Check your understanding using the graphing feature at right, Current vs battery voltage.
  4. Change the wavelength back to 400nm and the intensity to maximum, then experiment with the battery voltage. Why does the current NOT depend upon the voltage when it is positive, but DOES vary when the battery is negative? What is special about the -0.80 V setting? (Question assumes "Sodium" & "400nm").
  5. Find the "stopping voltage" for each of several different "colors," including UV wavelengths. Compute the frequency for these colors using f = c/lambda. Graph stopping voltage vs frequency; compare to the graph at right, "Electron energy vs light frequency" to see the related graph..
  6. Regraph "Electron energy vs light frequency" using the applet grapher for several different metals. How does this argue for the photon theory of light?