# Homework 13 Question 4

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Two light sources are used in a photoelectric experiment to determine the work function for a particular metal surface. When green light from a mercury lamp (λ = 546.1 nm) is used, a stopping potential of 0.710 V reduces the photocurrent to zero.

I understood part A but can't seem to figure out part B:

(b) What stopping potential would be observed when using light from a red lamp (λ = 690.0 nm)?

I've tried using the equation Kmax = e*V and solving for V. When first solving for Kmax, using Kmax = E - W, I found E by diving 1240/690nm to get 1.797 and have been using 0.734 as my W as that is the max kinetic energy of electrons. Once I have Kmax, I've tried a lot of different operation with 0.710 V and can't seem to get the right answer. Where am I going wrong?

Based on your calculation in part (a), the work function should be 1.56 ev.  $eV_s=hf-W$.
Using this work function, you can find the new stopping voltage with a different wavelength. $eV_s=hf-W=1.797-1.56$.