Hello everyone, I know that the photon has zero rest mass and I know that otherwise the relativistic mass formula would not make sense. When I searched for an answer to the question "how can a particle have zero mass and still possess energy if E=mc^2" I got an answer saying the mass energy relation formula is actually E= m^2 c^4 times p^2 c^2 when we talk about a non massive particle. Back then I diddn't think about p=mv formula has a mass in it. The problem started when our physics teacher argued that although photons doesn't have rest mass, they still have a relativistic mass. We transformed these formulas E=mc^2 and E=hf to give us the photon mass m=h/λc. The teacher also said that the photons have momentum, and we can see it in the deflection of the halley comet near the sun, and that we can even build a solar sail that uses the momentum of the photons (he diddn't mean solar cells). So my knowledge about photon mass is contradicting with itself. Where am I wrong?