Consider an excited hydrogen atom in state n moving with
Question:

Consider an excited hydrogen atom in state $n$ moving with a velocity $v(v<c)$. It emits a photon in the direction of its motion and changes its state to a lower state m. Apply momentum and energy conservation principles to calculate the frequency $v$ of the emitted radiation. Compare this with the frequency $v_{0}$ emitted if the atom were at rest.

Solution: