1. The problem statement, all variables and given/known data Deduce an approximation for Planck's radiation law: To what what wavelength does hv/kT=1 2. Relevant equations Wien Distribution equation B(lambda)= 2hc^2/lambda^5*exp(-hc/kT(lambda)) Rayleigh-jeans approximation B(lambda)=2ckT/lambda^4 3. The attempt at a solution My main concern is with the latter part of the question: To what wavelength does hv/kT=1? My first attempt at finding a wavelength was to equate Rayleigh-jeans approximation equations and the Wien Distribution equations to each other. (2hc^2/lambda^5)*exp(1/lambda) = 2ckT/lambda^4 . Then to eliminate T from the equation , I would set T= hv/k. Therefore 2ck(hv/k)/(lambda)^4 = 2hc^2/(lambda)^5*(exp(-1/lambda) => lambda=c/2/v. The only problem is , I still have an unwanted unknown, which is the frequency. Do you need the frequency in order to determine the wavelength?