Consider the following.
(a) For a hydrogen atom making a transition from the n = 4 state to the n = 3 state, determine the wavelength of the photon created in the process. (Already solved this, 1.86×10^3 nm)

(b) Assuming that the atom was initially at rest, determine the recoil speed of the hydrogen atom when it emits this photon.

2. Relevant equations

λ=hc/ΔE

3. The attempt at a solution

I’ve done one like this previously, but we were given the energy of the photon colliding with the atom. Found the Kinetic Energy, set it equal to 1/2mv^2 and solved for v. I don’t think I can do this for part b. I think this has something to do with momentum perhaps? I’m kind of lost on this one.