1. Photon runs into motionless electron and gives the electron all of its energy 2. Fast positron interacts with motionless electron producing one photon 1. I know that if that electron were in atom, photon of some energy could give its energy, and electron would jump into a higher energy state. But what about free electron, as in this case? And why not if it's not possible? 2. positron and electron should anihilate producing photon, but I'm not sure is it 1 or two photons, and I don't know the argument why.