Finding dn/dλ from n vs 1/λ² fitting equation at 800 nm

[mufc4ever]

Homework Statement

Determine the dispersion (dn/d lambda) at a wavelength of 800 nm from fitting equation.

Homework Equations

Fitting equation from graph of n vs 1/lambda^2

y = 7e-15 x + 1.60

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?

 

[rude man]

Looks like you went dn/d(1/λ²) = (dn/dλ)(dλ/d(1/λ²)
= (-2/λ³)dn/dλ
which is what I would have done.

That math has to be correct, so you probably erred somewhere along the line in trying to fit a new curve of n vs. λ. That curve of course should look close to a straight line ...

How about giving us a few numbers you computed?