1. The problem statement, all variables and given/known data Find the wavelength of an electron which is travelling at 4.35*10^6 m/s. 2. Relevant equations p = h/λ p = mv E = hf E = 1/2mv^2 3. The attempt at a solution I know this can be easily solved using the momentum equation and De Broglie's law like this: mv = h/λ (9.109*10^-31)*(4.35*10^6)=(6.626*10^-34)*λ λ ≈ 0.167nm But here comes the actual question... Why can't I solve this with the second law E = hf and the classical 1/2mv^2? 1/2mv^2 = hf, where f = (c/λ) This gives me an incorrect result. If I wanted to use kinetic energy, I would have to first convert it to momentum p = √(2Em), which I would use with p = h/λ. After all, for instance the photoelectric effect can be calculated using kinetic energy with hf-W. I think I've missed something relevant, and I can't seem to find the answer. Sorry if this is too obvious.