Ok guys got a question on quantum physics - photoelectric effect i wanna run past you before i attempt a solution. The question is as follows; In a photoelectric-effect experiment, light of wavelength 420nm is incident on a sample of sodium. The work function of sodium is 3.67*10-19 J. Calculate the stopping potential required to stop all electrons from reaching the anode. I dont really know where to start as im not completly sure on what the question is asking for. I know how to work out the maximum kinetic energy of the ejected electrons by way of hf-work funtion, but i KNOW this isn't the way to approach it. Any ideas or suggestions?