What is the formula for the width of the potential box?

[dawn_pingpong]

Homework Statement

An excited electron (quantum number =4) in a 1D potential box de-excites to the ground state and emit a photon. This photon subsequently becomes absorbed by an electron in a hydrogen atom. The electron in the hydrogen atom becomes excited to an excited state with quantum number =3. The process is illustrated in the following diagram. Determine the width of the potential box L.

Homework Equations

n(subscript n)=-13.6/n^2 eV

or frequency= 3.29x10^15 x Z^2(1/nfinal-1/ninitial) then use E=hf. I actually don't really understand when to use which equation. Can someone kindly explain to me?

Also, what is the formula to calculate the width of the particle in box box?
Thank you very much!

The Attempt at a Solution

 

[voko]

Think about the energies of the original electron, the photon it emits, and the hydrogen atom's electron.