What is it about photons that their sub-threshold energy is dissipated but that of waves is not?
The idea is this: with photons, which are chunk-like bits of energy, an electron will be kicked out if there’s enough energy, but won’t be if the photon has less than the threshold energy. If the photon doesn’t have enough energy to kick out an electron, it will either keep on going, or interact and deposit energy in some other way, and its energy will perhaps end up as heat or whatever (energy must always be conserved). In the wave picture, the waves come along and wiggle electrons. Their energy could indeed be partly dissipated in the material as heat or whatever; I did not say it couldn’t be. But the difference in this picture is that a wave can keep continuously wiggling an electron, gradually transferring more and more energy to it, until finally the electron gets enough energy to break free. The key difference is that with a wave, there’s no threshold effect: long term action of a low energy wave can eventually give an electron enough energy to pop out. In the photon picture, too
