Department of Physics and Astronomy
Points of Emphasis
A. In Part A of the lab the students plot the large scale retarding voltage vs. photocurrent curve for a mercury lamp-phototube system using a single frequency of light and three different intensities. The intensity of the light is varied by positioning the phototube different distances from the mercury lamp. From the plot, the students see that:
This part also serves as an introduction to what they are going to be doing in part B.
B. In Part B the students will measure the stopping potentials Vo for four frequencies of light. They are then asked to plot eVo vs. n and compute, from the slope of the line and from the y-intercept respectively, an experimental value for Planck's constant h and for the work function of the anode f . The experimental values for h and f will then be compared with their accepted values. From their plots of eVo vs. n , the students will see that:
Introductory: P19, P24
1. The quantitative results of this lab are not tremendous. Expect results to vary from theoretical values by ± 20% or less.
2. The plotting pens are now a specialty item and cost ~$7.00 apiece. If the caps are left off overnight, they will often dry out and need to be replaced. This can get expensive if the students are not careful about putting the caps back on.
3. Please make certain all of the electrical equipment is turned off after the lab. Pay particular attention to the Keithley nullmeters. They run on batteries which are expensive to replace and we do not keep many spares on hand.
4. Even when the mercury tube-water lens-phototube system are all lined up in a straight line, the light exiting the water lens may not fall on the phototube window. Students should visually check to see that the focused light is indeed falling on the phototube before attempting any measurements. If a student complains that his system does not work, this should be one of the first things to check. The easiest way to fix this problem is to rotate the mercury tube until the light falls on the phototube window.
5. The signals from the phototubes tend to be noisy and the response of the x-y recorder to rapidly changing signals is poor. Therefore, it is necessary to turn the helipot dial very slowly when plotting the photocurrent vs. retarding voltage curves. This is particularly true in part B of the lab. This will give the plotter time to respond in a relatively smooth fashion to the incoming signal. Patience is needed to get good plots.
Part A - Plot of Photocurrent vs. Retarding Voltage (-5.0 V to +5.0 V)
Part B- Plots of Photocurrent vs. Retarding Voltage (-5.0 V to +0.0 V) for the four filters.
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