# Emitted Photons

1. Sep 21, 2008

### jesuslovesu

1. The problem statement, all variables and given/known data
A hydrogen atom has its electron in the n = 2 state. The electron makes a transition to the ground state. The linear momentum of the emitted photon is E/c. If we assume conservation of linear momentum, what is the recoil velocity of the atom?

2. Relevant equations

$$v_n = \frac {e^2}{2 \epsilon_0 h n}$$

3. The attempt at a solution

Momentum Before:
$$p_i = m_e v_e$$
where $$v_e = \frac{e^2}{4 \epsilon_0 h }$$

Momentum After:
$$p_f = E/c + m_? v_{recoil}$$

My question is: Is this analysis correct? and
For Bohr atoms do what mass do I use (mass of a proton and a neutron, or just a proton?)

I am questioning the way I set this up because I don't know what the nucleus contains (proton and neutron or just a proton).