Solve Photoelectric Effect: Find Work Function & Planck's Constant

[patapat]

Homework Statement

When light of wavelength 450 nm is shone on potassium, photoelectrons with stopping potential of 0.52 V are emitted. If the wavelength of the incident light is changed to 300 nm, the stopping potential is 1.90 V. Using only these numbers together with the values of the speed of light and the electron charge, do the following.
(a) Find the work function of potassium
(b) Compute a value for Planck's constant.

Homework Equations

eV_{0}=hf-\phi

The Attempt at a Solution

I'm not sure how to do this problem. Since they ask us to compute a value for Planck's constant I am not sure that we actually use the equation stated above. I assume that the work function is the same for both cases of wavelength and stopping potential, but I'm not sure what the relationship is.
Thanks in advance for the help.

-pat

 

[patapat]

it should be V_0 not V^0, thanks.

 

[proton]

You can start with (b) and then do (a). Have two equations of (phi) = hf - eV_o, and set them equal to each other since they each the same phi value. You should then be able to solve for h. Then plug h into one of the equations to find phi