The momentum of the electromagnetic (em) fields Pe appears in several areas of modern physics. In both the equations for matter and light wave propagation Pe represents the relevant em interaction. As an application of wave propagation properties, a first order optical experiment which tests the speed of light in moving rarefied gases is presented. We recall that Pe is also the link to the unitary vision of the quantum effects of the Aharonov-Bohm (AB) type and that, besides the traditional classical approaches to the limit of the photon mass mph, effects of the AB type provide a powerful quantum approach for the limit of mph. Table-top experiments based on a new effect of the AB type, together with the scalar AB effect, yield the limit mph = 9,4?10?52 g, a value that improves upon the results achieved with other approaches.
