The photoelectric effect cannot be understood within the wave picture of light. To eject an electron from a metal surface a certain amount of energy f , called the work function of the metal, must be supplied to this electron. In the wave picture the energy of the light beam does not depend on the frequency, but only on the intensity. Einstein explained the photoelectric effect by postulating that an electron can only receive the large amount of energy necessary to escape the metal from the EM wave by absorbing a single photon. If this photon has enough energy, the electron is freed.
Excess energy appears as kinetic energy of the electron.
If the photon does not have enough energy, then the electron cannot escape the metal. Molybdenum has a work function of 4.2eV. Find the cutoff frequency and cutoff wavelength for the photoelectric effect. J is the workfunction of the surface. Hz is the cutoff frequency. In a homogeneous, isotropic medium light travels in a straight line.