For a plane light wave in a uniform medium, observed in the medium-rest frame, the Abraham momentum formula_38 is equivalent to the Planck's momentum formula_39 with formula_40 the energy flux (= energy density multiplied by velocity), usually called Plank's principle or Planck's theorem. According to Ives, the Planck's momentum was first (implicitly) derived by Poincarè in 1900, and later (in 1907) Planck utilized it to study the relation between inertial mass and quantity of heat for a body. Since formula_41 is Einstein's mass-energy equivalence equation, the Planck's principle is essentially the same as the Newton's law (momentum = mass multiplied by velocity), which is often used in solving the Abraham–Minkowski problem, such as in Leonhardt's analysis based on the wave–particle duality of light, and in Barnett's analysis based on Einstein's medium-box thought experiment.