# Griffiths Quantum Question. Unit-less Schrodinger Equation.

## Main Question or Discussion Point

In Grifftiths Intro to Quantum 2nd edition, page 51, he is re-expressing the Schrodinger equation for a harmonic oscillator in terms of a unit-less quantity $\xi \equiv \sqrt{\frac{m\omega}{\hbar}}x$

So Griffiths takes the Schrodinger equation in equation [2.70]
$-\frac{\hbar^2}{2m} \frac{d^2\psi}{dx^2} + \frac{1}{2}m\omega^2x^2\psi =E \psi$

and then re-expresses it in equation [2.72] as
$\frac{d^2\psi}{d\xi^2} =(\xi^2-K)\psi$

where $K\equiv\frac{2E}{\hbar\omega}$

My question is I am trying to follow what Griffith is doing here and was wondering how to re-express the 2nd derivative of $\psi$ going from $\frac{d^2\psi}{dx^2}$ to $\frac{d^2\psi}{d\xi^2}$?

Is it just by using a bunch of chain rule stuff?

For example, using
$\frac{d\psi}{dx} = \frac{d\psi}{d\xi} \frac{d\xi}{dx}$
then
$\frac{d^2\psi}{dx^2} = \frac {d}{dx} \Big( \frac{d\psi}{dx} \Big) \\ \frac{d^2\psi}{dx^2} = \frac {d}{dx} \Big( \frac{d\psi}{d\xi} \frac{d\xi}{dx} \Big)\\ \frac{d^2\psi}{dx^2} = \frac {d}{d\xi} \Big( \frac{d\psi}{d\xi} \frac{d\xi}{dx} \Big)\frac{d\xi}{dx}\\ \frac{d^2\psi}{dx^2} = \Big( \frac{d\psi}{d\xi} \frac{d}{d\xi}\Big(\frac{d\xi}{dx}\Big) + \frac{d\xi}{dx} \frac{d}{d\xi} \Big(\frac{d\psi}{d\xi}\Big) \Big)\frac{d\xi}{dx}\\ \frac{d^2\psi}{dx^2} = \Big(\frac{d\xi}{dx}\Big)^2 \frac{d^2\psi}{d\xi^2}\\ \frac{d^2\psi}{dx^2} = \frac{m\omega}{\hbar} \frac{d^2\psi}{d\xi^2}\\$

Does this look legit?

Yes. $m\omega\over \hbar$ is constant.