1. The problem statement, all variables and given/known data This problem has a photon that was emitted by an electron orbiting a hydrogen atom that dropped from n=5 orbit to n=2. I need to find out, if this photon emitted an electron from the n=4 orbit of another hydrogen atom, what the k.e. of this electron would be. 2. Relevant equations 1/λ=R(1/n_f^2-1/n_i^2) E=hc/λ not sure of what else i need 3. The attempt at a solution So far I found the wavelength using the first equation I listed, and then from this found the energy of the first emitted photon. I got 2.856 eV. I'm pretty sure im supposed to use this information to find out what the k.e. of the electron that this photon emits is, but im unsure of how to do that. At first I thought maybe the energy of the photon was equal to the total energy of the electron, and so 2.856 eV=k.e.+rest energy of the electron, but this obviously didnt work (got a negative number for k.e.). Im sure theres just some equation Im missing. If someone would fill me in that would be appreciated.