1. The problem statement, all variables and given/known data Compute the wavelengths in fused silica and in silicon for photons whose free-space wavelengths are 800 nm, 1300 nm, and 1550 nm. 2. Relevant equations This is the problem, I'm not sure what equation to use and the textbook does not seem to say, well what little I have of the text because I was forced to take pictures from a classmate's book. 3. The attempt at a solution I have a feeling the equation might be ω0=(λ f)/( ∏ω) but this is under a section about Gaussian light-beams, something the problem makes no reference to anyway. I just need the equation and I can solve the rest from there.