1. The problem statement, all variables and given/known data A ray of photons coming from a laser with a wavelenght of 589.59nm meets a ray of sodium atoms (A=23) coming from the opposite direction. What is the maximum speed the sodium atoms can have in order to absorb photons? 3. The attempt at a solution I thought about viewing this as a kind of photo-electric effect, the work function of sodium being 2.46 eV, but then I realised the problem is about absorbing photons, not about emitting photo-electrons. The energy of the photons seems relevant: E = hc/λ = 3.369*10^-19 J As this is less than the work function for sodium, I am pretty convinced I shouldn't be viewing this as a photo-electric effect problem. I was also thinking the clue might be relativistic energy, viewing the sodium atoms and the photons as two frames of reference. Is there perhaps a kind of maximum (kinetic) energy a sodium atom can have if it wants to absorb a photon? Am I perhaps supposed to bring into account different energy levels of sodium with different quantum numbers?