1. The problem statement, all variables and given/known data Estimate the wavelength of a 30 keV electron. 2. Relevant equations p = h/λ E = hf 3. The attempt at a solution The electron has 30 keV of mass-energy. As it is acting as a particle-wave, E = hf, so f = E/h = 30,000x1.6x10^-19/6.3X10^-34 = 7.619x10^18 Hz Now, v = fλ, so λ = v/f, but this is where I run into a problem, as I'm not told what speed the electron is travelling at - do I assume speed of light, or is there some other way?