# Spectroscopy question

1. Jan 14, 2008

### neelakash

1. The problem statement, all variables and given/known data

For a many electron system with L=2,S=1 and J=2 calculate the angle between the L and S vectors both according to the old quantum theory and quantum mechanics.

2. Relevant equations

3. The attempt at a solution

I suppose this problem is nothing but a simple application of cosine rule.The external angle is the answer.
But I am thinking twice as they mention: both according to the old quantum theory and quantum mechanics.

Am I missing something?

2. Jan 14, 2008

### Tom Mattson

Staff Emeritus
In the Bohr model (I think that qualifies as "old quantum theory"), we have $L=n\hbar$, where $n=1,2,3,...$. Contrast that with modern quantum mechanics.

Does that help?

3. Jan 14, 2008

### pam

In QM, use LS cos(theta)=L.S=[J(J+1)-L(L+1)-S(S+1)]/2.