GOALS

Physics

• Measure Planck's constant by using the photoelectric effect.

Technique

• Use the oscilloscope and a vacuum photodiode to find "cut-off" energy of photoelectrons.
• Appreciate the "reverse" current problem in the photodiode.
• Graph photo-electron "cut-off" energy vs. photon frequency in a way which shows the "work function" and allows you to find Planck's constant h.

Errors

• Use allowable slope fits to your data points to estimate the accuracy of h.
• Compare your value of h to the accepted value.

Question (work out in lab book before lab)

1. Suppose that you have applied 1 volt between the anode and cathode of the photodiode and that 99% of the electrons from the cathode are prevented from getting to the anode. If there are 100 times as many electrons being kicked out of the cathode as there are from the anode, what is the anode current which you see on the oscilloscope)? Does this correspond to the "cut-off" voltage?

References

• Weidner and Sells, 4-2.
• Bueche, 35.3.