This is a question about transforming a probability distribution, using the blackbody spectrum as an example. The problem statement, all variables and given/known data An opaque, non-reflective body in thermal equilibrium emits blackbody radiation. The spectrum of this radiation is governed by B(f) = af3 / (ebf−1) , where a and b are constants and f is the frequency of the emitted light. Work out the corresponding distribution of wavelengths B(λ) using f = c/λ . The attempt at a solution I tried substituting f = c/λ into the given equation - which seemed like a good place to start - and came out with the following: ac3 / λ3(ebc/λ - 1). Then when I looked up the actual spectrum in terms of wavelength (on HyperPhysics, here) it gives something else. This is the HyperPhysics formula: 8πhc / λ5 ⋅ 1 / (ehc/λkT - 1) Comparing the constants in the frequency spectrum on HPhys with a & b in the formula in the question: a = 8πh / c3 b = h / kT Which then turns my answer of ac3 / λ3(ebc/λ - 1) into: 8πh / c3 ⋅ c3 / λ3(ehc/λkT - 1) = 8πh / λ3 ⋅ 1 / (ehc/λkT - 1) It looks like I'm out by a factor of c / λ2 compared to the 'real' wavelength spectrum - have I approached this the wrong way, or has the question just simplified the given formula in some way?