I am extremely confused by the use of the term coefficient of increase of something. For example , if it is stated that the index of refraction varies linearly with a coefficient of 2.5×10^-5, how is this coefficient defined? Is it simply the slope of the line plotted with index of refraction on y axis and temperature on x axis? Or is it defined like in the case of coefficient of linear expansion? α = Δn/ΔT or α = Δn/nΔT? P.S. I am not sure if this falls under mathematics or physics. Excuse me if I posted it on the wrong topic.