Hi, Probably a really stupid question... but I'm confused about how to relate a line width in frequency to one in wavelength. To me it seems obvious that if there's a broader spread of frequencies, there must be a broader spread of wavelengths, and vice versa - after all, the line is broader. But... if you relate Δf and Δλ mathematically (f is frequency, since there was no nu on the quick symbols list): c = fλ f = c/λ df/dλ = -c/(λ^2) so df = dλ * (-c/(λ^2)) I.e. if df is positive, dλ is negative... What am I doing wrong??