Hello all, I'm in the process of reading a few books on quantum mechanics and have come to a few points that I don't fully understand. Hopefully some of you guys can clear this up. Ok quickly stated, photo-electric effect shows how light can act as a particle because of the fact that there is a threshold at a certain higher energy light that can change the charge of the plate (ie UV light works and visible light doesn't). But I am confused on the core concept of how photons can have higher or lower energies. If the speed of light (photons) is constant and they are known to have no mass, then how can they have different energies?