# Homework Help: Raising and lowering operators / spherical harmonics

1. Nov 26, 2007

### nowits

This isn't exactly a part of any problem, but a part of a generic principle. I don't understand the use of raising and lowering operators.

$$L_{^+_-}=\hbar e^{^+_- i l \phi}({^+_-}\frac{\partial}{\partial \theta}+ i cot \theta \frac{\partial}{\partial \phi})$$

So how does one use $$L_{^+_-}Y_l^m$$ to gain $$Y_l^{m{^+_-}1}$$

2. Nov 27, 2007

### malawi_glenn

you can use this definition to apply it to the general expression for the spherical harmonics. The general expression for an arbitrarty spherical harmonics can be found either in your textbook, or google it.

3. Nov 27, 2007

### nowits

Do you mean that I simply:
$$\hbar e^{^+_- i l \phi}({^+_-}\frac{\partial Y}{\partial \theta}+ i cot \theta \frac{\partial Y}{\partial \phi})$$

But what happens to hbar? There isn't supposed to be any hbars in Y's?

4. Nov 27, 2007

### malawi_glenn

it is just a constant..

5. Nov 27, 2007

### malawi_glenn

Did you get it? $$\hbar$$ is only a multiplcative constant, same as $$e^{^+_- i l \phi}$$. So you find out what the derivative operators does on the general spherical harmonic.

6. Nov 27, 2007

### nrqed

applying, say $L_+ Y^l_m$ does not give $Y^l_{m+1}$, it gives a constant times $Y^l_{m+1}$. You may find the general constant in eq 4.121 of Griffiths, for example.

7. Nov 27, 2007

### nowits

Yes, I think I understand it now.

Thank you both.