(2π)f = E = m, derived from: E = mc^2 E = hf therefore, hf = E = mc^2 To simplify and avoid precision/rounding errors it's desirable to express constants as natural ratio: ħ = h / 2π (reduced Planck constant) h = 2πħ h = 2π, where ħ = 1 Therefore, E = (2π)f