# Need help with modern physics problem

1. Apr 19, 2009

### okfine444

1.Question Its due tommorow and its a former AP Physics B free response questions. I appreciate any help. It comes with a diagram which I had to put in a different link.

Diagram:
http://img.photobucket.com/albums/v3...1/physics1.jpg [Broken]

Question:
Electromagnetic radiation is incident on the surface S of a material as shown above. Photoelectrons are emitted from the surface S only for radiation of wavelength 500 nm or less. It is found that for a certain ultraviolet wavelength, which is unknown, a potential Vs of 3 volts is necessary to stop the photoelectrons from reaching the anode A, thus eliminating the photoelectric current.
a. Determine the frequency of the 500 nm radiation.
b. Determine the work function for the material.
c. Determine the energy of the photons associated with the unknown wavelength.
d. Determine the unknown wavelength.
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2. Relevant Equations E=hf f=c/lambda and I'm sure there is more but I cant figure out the work function.

3.Attempt at a solution I've honestly only done part A and Im not even sure if it's correct. i got f=(3x10^8m/s)/(5x 10^-7m)= 6X 10^14 s^-1. I cannot figure out the work function for the material and whats really throwing me off is the 3 volts necessary to stop the photoelectrons from reaching the anode; I honestly don't know how to incoporate that into the problem. Any help is greatly appreciated!

Last edited by a moderator: May 4, 2017