Question

When we say that a photon is a “quantum of light,” what does that mean? What is quantized?

Answer 1

It means that each photon can only a certain amount of energy associated with it. This certain amount of energy can not be subdivided further. let us say, we can not have half of the energy possessed by a photon. But we can have multiples of this energy possessed by several photons. Each photon has a characteristic energy and is emitted when a transition equivalent to that energy occurs in an atom.