I'm a second year physics student and we've been talking about light and the fact that it carries momentum. I've been thinking about a situation where there is an excited atom that has an electron at a high energy level. When the electron jumps back down to a lower energy level, it will release a photon. This photon has momentum and according to the conservation of momentum, the atom must gain the same amount of momentum in the opposite direction. I understand that the light is released and it immediately is moving at c, with momentum p = h/λ. This means that the momentum of the atom must be mv=-h/λ.
What's bugging me, is that the photon isn't accelerated, it has this momentum immediately when it is emitted, which means the atom must also have this momentum (but negative) when the photo is emitted. Does this mean that the atom goes straight from 0 velocity to a non-zero velocity without accelerating? Or is something else going on that I don't know about?