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.