This is more of a refresher of me, but I'm wondering how to formally approach a simple substitution of variables.
I want to substitute $\omega$ for $\lambda$ in a simple derivative:
$\frac{dn}{d\omega}$ to $\frac{dn}{d\lambda}$, when $\lambda = 2\pi c / \omega$.
I know it is $\frac{dn}{d\omega}$ = $(-\lambda^2 / 2\pi c)\frac{dn}{d\lambda}$
and that is achieved by simpling realizing that the derivative of what is given will cancel out the multiplicative constants, but I can't remember what that process is called and feel like a bit of an idiot!