The quantum molecular dynamics method is applied to understand the fundamental mechanism of light-to-heat conversion in atomic systems under light irradiation. The light energy governs the time history of kinetic energy of the atomic system under light irradiation. Under infra-red light irradiation, the atomic fragments made by light irradiation have translational velocities parallel to the direction of light fluctuation. Under light irradiation at electron energy levels, the fragments tend to be isolated atoms having random translational velocities. The light electric field interacts with the atomic system through different terms of the molecular dynamics equations in the thermal conversion processes of light in the visible to infra-red range. Under infra-red light irradiation, the light interaction comes through fluctuations of effective dipole moment, whereas light irradiation at electron energy levels, it comes from changes in the potential energy between atoms associated with the electronic excitation.