an electron travels at 1 x 106 m/s. when it hits a target, its speed decrease in half and a photo is released. find the phton's wavelength. so far i know E photon = (hc) / lamda what about the speed? does the photons' energy same as kinetic energy of the electron which is just 1/2 mv ^2? if so, do i use the halved speed or the initial?