How can a photon knock an electron from a shell, entirely out of the atom? (i.e PhotoElectric Effect) Photons have 0 rest mass, and little relativistic mass? For force, F = ma For example, let m = negligable (like the photon's mass is) mass so = mn If F = (mn)a and mn is small, then no matter how fast the acceleration, in this case c = 3.0*10^8ms-1, Force will still be negligable. Yes, it is true that you cannot use classical mechanics and apply it directly to quantum physics, in this case it would be better to use relatavisitic calculations, but most formula's in QM were developed from classical motion formulas. Just take the bohr radius for example, its a contruction centripetal acceleration (classical circular motion), F = ma is even interpolated in it, Couloms Law and other formula's. This gives the bohr radius 0.0529 * 10^ -10m I propose that an electron can only absorb or reflect a photon... If it absorbs the photon, the electron will go into an exicted state, i.e its energy will be greater, and it will leave the discrete quantised energy level which requires the electrons to be at a certain energy. Both wavefunctions for electrons and photons are on the scale of nano meters or 10^-10 m. Is there a correllation between the W0 (work function) proposed by Einstien, - i.e the minimum energy needed to knock an electron out of an atom, and the wavelength of an electron in a particular energy level to the wavelength of the incident photon that has W0? Maybe wavefunctions of the same frequency or wavelength determine wheter a photon is absorbed or reflected? And why do we talk of photons as is they are comparable in anyway to a fermion or hadron? Like when we say when an electron jumps from a higher shell to a lower shell, it will emit one red photon for example.