What is the Quantum theory of light in an intuitive explanation?
The quantistic theory of light is based on its dual interpretation wave-particle because it is an obligation of experimental evidence.
In fact the light shows both characters of waves or particles depending from the observation mode we can apply. If you let interact the light with an optical system as a mirror, it will response as an ordinary wave with reflections, rifractions and so on. Conversely, if you let interact the light with external bonded electrons of an atom, they can be pushed out of their orbitals as in a "balls" collision process (photoelectric effects).