Photons with 605 THz frequency strike metal of 1.2eV work function. Calculate the maximum energy of photoelectrons and their velocity. What amount of energy is necessary to stop all photoelectrons? (Planck's constant. electron mass and charge are given)

Start off by converting all given data into SI units to avoid confusion later on. The photoelectric effect happens when photons with energy larger than work function strike metal, thus releasing the electrons from it. Write down the formula for photoelectric effect and check if all terms are understood: KE = E(photons) - W = hf - W (W- work function, KE- kinetic energy, f- frequency of photons). Put in the values to find kinetic energy: KE = 2.0810^-19 J. For velocity just adjust the kinetic energy formula from KE=(mv^2)/2 to v=(2KE/m)^1/2. Plug in the values and find the velocity v = 675752.2 m/s. As for the last part, it is enough to use the same amount as maximum kinetic energy to completely stop all photoelectrons. The stopping energy must be higher or equivalent to the photoelectron energy.