If you shine photons onto a sheet of a metallic plate with say, a very dim light. You might expect that electrons in orbit in atoms in the metal will keep absorbing energy and eventually they would have enough to fly out of orbit....
THIS DOESN'T HAPPEN.
What ACTUALLY happens is... Nothing. If you put more bulbs... Nothing happens.. .... 1000's of bulbs with the same frequency.. nothing happens.
It was discovered that if you start increasing the frequency, as soon as the photons hit the surface of the metal, electrons start flying off immediately.
This suggests, electrons only absorb "spoonfulls" of certain amounts energy or quanta. Which SUGGESTS that the photon is a distinct bundle of energy, similar to a particle.
I presume you're familiar with the bohr model?
Electrons exist in an atom at distinct energy levels.