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## Homework Statement

I have a test in my physics class on tuesday, and as a study guide, our professor gave us a sample test and the solutions. I worked through most of it, except for this one question. I don't understand his method to solving it, I was hoping you guys had a solution. I have already email him about it, but he has yet to answer.

When an electron decays from the n=2 state to the n=1 state, it emits a photon. Determine the size of the box, L, for which the energy of this photon equals the energy of the analogous n=2 to n=1 transition in hydrogen.

## The Attempt at a Solution

HIS attempt at the solution is in the attached jpeg.

I understand the process of finding the energy of the photon, that's very easy, but in finding the size of the box, he introduces this pi

^{2}term, and instead of doing the 1/n

^{2}- 1/m

^{2}, he simply does n

^{2}-m

^{2}