The classical form of the energy conservation law (and in fact the notion of energy in the first place) is directly related (through the corresponding equation of motion) to the force- concept describing the interaction of particles. The latter can be shown to be necessarily instantaneous (i.e. Newtonian) as otherwise one would not be able to define a force objectively, i.e. independent of the state of motion of the observer. One can therefore say that the law of energy conservation does, by definition, only strictly hold for this case of a static interaction of particles, but is not more than an arbitrary ad-hoc concept if applied to other situations, in particular those involving light: two light waves can be made to extinguish each other completely if superposed with the correct phase, which proves that a form of energy conservation does not apply here. There is also theoretical and observational evidence that the conversion between atomic and radiative 'energy' can not be described by a unique constant but is variable depending on certain physical parameters .
(Note: some physicists claim that a general law of energy conservation derives from the so called 'Noether's Theorem'. This is a misinformation as Noether's theorem utilizes Lagrangian functions which in turn contain potential energy functions which in turn can only be defined for conservative force fields, i.e. for Newtonian physics in the sense as described above)