Hi everyone, I just have some confusion regarding Planck's and Einstein's equation. The following is an explanation of the photoelectric effect using Einsteins theory: Light is composed of photons. Each photon has energy hf and mass hf/c^2. When ultraviolet photons are brought to rest by zinc, the mass of the photon changes to energy. This energy is used to break the binding energy between the outermost electron and the nucleus. The excess energy is carried by the electrons as kinetic energy. Here is my confusion though: When we ask for the energy of a photon, we say it is equal to E = hf. When a photon is brought to rest in the explanation of the photoelectric effect, it says the mass of the photon during its motion changes to energy. So shouldn't the energy of the photon be hf/c^2, not hf? For example, we say that the kinetic energy of the electrons in the photoelectric effect is: KE = hf - w where w is the work function. How come it isn't KE = hf/c^2 minus w Maybe I am misunderstanding the photoelectric effect when I say that the mass of the photon changes to energy? Perhaps the mass does not completely change into energy, and thus hf/c^2 is not applicable, only for E = mc^2? If my question is unclear, let me know. Thank you very much.