*Edit: Nevermind, I realize where I went wrong! 1. The problem statement, all variables and given/known data The following graph shows the kinetic energy of the most energetic photoelectrons as a function of the frequency of light falling on the cathode in a photoelectric cell. http://img208.imageshack.us/img208/3421/fwby.jpg [Broken] Use the graph to determine the value of Planck's constant. 2. Relevant equations E = hf h = E/f 3. The attempt at a solution I'll choose the point where E = 3.0 eV and f = 10.0 Hz for my calculation. First I'll convert eV to Joules: 3.0 eV x (1.60 x 10^-19 J / 1 eV) = 4.8 x 10^-19 J Now the calculation: h = E/f h = (4.8 x 10^-19 J) / (10.0 x 10^14 Hz) h = 4.8 x 10^-34 J(s) We already know that the value of Planck's constant is 6.63 x 10^-34 J(s), so my answer is off. Any suggestions?