Hi, I got the following question in my textbook: [translated]"Compare the wavelength of a photon and an electron where the photon and the electron have the same momentum". My thinking is the following: Firstly, pp (photon) = pe (electron). My textbook briefly mentions the extention of the mass-energy equivalence E2 = p2 c2 + m2 c4, so I go by this since the particles have different speeds. The de Broglie wavelength of the electron is given by fe = E/h = √(p2 c2 + m2 c4) / h. The wavelength of the photon is the same except that the mass is 0, so it reduces to fp = √(p2 c2) / h. Since m2 c4 ≥ 0 it follows that fe ≥ fp. My textbook says that the answer is that fe = fp tho. Their argument is that p = h/λ holds for both the electron and the photon. But they previously state that it only applies for massless particles. They derive it from E2 = p2 c2 + m2 c4 by setting m to 0. [translated]"[...]E = pc which holds for massless particles.[...] we find that p=h/λ". Why is my argument invalid and why does λ=h/p hold for the electron? Thanks!