In Classical Mechanics the derivative of Kinetic energy with respect to velocity is momentum, so I was wondering if this is valid: In this L is lambda or wavelength and h is Planck's Constant. E=hf, therefore E=h(v/L) This means that dE/dv=h/L.(h and L are constants and the derivative of v would be 1.) Rearranging this we get: L=h/p. I just wanted to know if this is considered valid.