1. The problem statement, all variables and given/known data "Absorption spectra of gaeous HCl show a line at 3.5 μm which results form molecular vibration (oscillation from interatomic distance)" How do I get the frequency of vibration from this? 2. Relevant equations ? If I knew this, then I could probably solve the problem 3. The attempt at a solution Is the wavelength 3.5μm? I don't see a relevant explanation in my book (An Introduction to Thermal Physics).