As we know electron and light have dual natures, they can be thought of as waves and as particles. The thing I never understood is why can you talk about say something like the angular momentum of an electron. In order for an electron to have momentum it needs to have mass. Yes, I know that the mass of an electron is found to be roughly 9.1x10^-31 kg, but when the electron appears as a wave what happens to its mass? Waves don't have mass, so what happens to its angular momentum? Is this why angular momentum comes in only discrete certain values? I want to know why you can describe certain things about electrons and light that depend on mass when electrons and light are not always particles.