# Quantum Mechanics (Wien’s displacement law)

1. Oct 3, 2015

### says

1. The problem statement, all variables and given/known data
Show that the maximum of the Planck energy density occurs for a wavelength of the form λmax = b/T, where T is the temperature and b is a constant that needs to be estimated.

2. Relevant equations
Planck energy density
u (v,T) = 8πv2 / c3 * hv / ehv/kT-1
3. The attempt at a solution

v= c / λ
dv = |dv / (dλ)| dλ = (c/λ2) dλ

I get up to this bit and I'm stuck...

v= c / λ
c = v*λ
∴ v = v*λ / λ
dv = dv*dλ / dλ
dv = (dv / dλ) * dλ

I'm confused how dv = |dv / (dλ)| dλ turns into (c/λ2) dλ. Sorry if my question is a bit vague -- haven't posted on here in a while and I just started taking QM.

2. Oct 4, 2015

### ehild

The absolute value marks are wrong in your equation.
d means differential, and dv/dλ is the derivative of v with respect to λ. d is not a multiplicative factor!
v is function of λ, and u(v,T)=F(v(λ),T). v=c/λ(v). You have to apply the chain rule to find the λ, where the u( λ) plot has its maximum:
dF/dλ = df/dv dv/dλ.
v=c/ λ, what is its derivative with respect to λ?

3. Oct 4, 2015

### says

v=c/ λ, what is its derivative with respect to λ?

dv = ( c / λ2 )

4. Oct 4, 2015

### says

v=c/ λ, what is its derivative with respect to λ?
dv = ( c / λ2 ) dλ

5. Oct 4, 2015

### ehild

No, the derivative is written as $\frac{dv}{dλ}$
In the derivative, you miss a minus sign.