**1. The problem statement, all variables and given/known data**

In order to achieve population inversion between two states that are separated by an energy difference of 3.5 eV, what wavelength of pumping should be used.

**2. Relevant equations**

Not sure.

**3. The attempt at a solution**

I have no idea where to start with this one. Can someone please point me in the right direction? There were no forumulas given. Thank you!!

http://ift.tt/1isq8Jk