1. The problem statement, all variables and given/known data Determine the dispersion (dn/d lambda) at a wavelength of 800 nm from fitting equation. 2. Relevant equations Fitting equation from graph of n vs 1/lambda^2 y = 7e-15 x + 1.60 3. The attempt at a solution Since it is a plot of n vs 1/lambda^2 I thought that x would be 1/lambda^2 so I took the derivative of that to get -2/lambda^3 and then put in the number. However I did not get a correct answer consistent with earlier findings. What am I doing wrong?