Determining Bond Length with Wavelengths

[PeachBanana]

Homework Statement

Calculate the bond length for the NaCl molecule given that three successive wavelengths for rotational transitions are 23.1 mm, 11.6, mm and 7.71 mm.

Homework Equations

I = h*wavelength/4pi*speed of light.

The Attempt at a Solution

I don't understand how to incorporate the wavelengths.
But the thing is, I have three wavelengths, not one. Should I take the difference between the three?

 

[ehild]

What are the allowed energy levels of a rotating diatomic molecule? How is the energy related to the moment of inertia? What are the allowed transitions?
Read http://hyperphysics.phy-astr.gsu.edu/hbase/molecule/rotrig.html

ehild