Calculating Kinetic Energy of Recoiling Hydrogen Atom

[patapat]

Homework Statement

A hydrogen atom at rest in the laboratory emits the Lyman radiation.
(a) Compute the recoil kinetic energy of the atom.
(b) What fraction of the excitation energy of the n = 2 state is carried by the recoiling atom? (Hint: Use conservation of momentum.)

Homework Equations

E=-13.6/n$$^{2}$$ev

The Attempt at a Solution

I'm not sure if this equation is relevant, but I'm not sure what they are referring to when they say recoil kinetic energy of the atom.

 

[Dick]

(Hint: Use conservation of momentum.) The departing photon carries momentum. The atom must have an equal and opposite momentum (hence a nonzero kinetic energy). The total energy of the two is the Lyman transition energy.