The mean energy per excitation wavelength of e.m. radiation propagated along a lossless uniform wave guide is found to bew = Qμ,Qbeing independent of frequency for a given amplitude of the longitudinal field intensity, mode of the field, and structure of the guide. The frequencyμand the group velocityvgof the radiant energy are related by the equationμ/μc = [1 − (vg/c)2]−12whereμc = c/λcdenotes the frequency at cutoff, andλcthe excitation wavelength at cutoff. This equation permits one to puthμ = hμc+hμc{[1 − (vg/c)2]−12 − 1}. The amplitudes of the field intensities are chosen such thatQequals Planck's constanth. Applying the mass-energy relation, the termhμc = hc/λc = m0c2may be interpreted as the “potential” (= rest) energy of the quantumhμandm0as the rest mass. The momentumpassociated withhμis found to bep = h/λg,λgbeing the wavelength along the guide. When radiant energy passes from one guide into another one of different cross-sectional structure via a tapered piece of wave guide, a partial conversion of “potential” to “kinetic” energy, and vice versa, occurs along the tapered section. In the case ofTEMwaves, which, however, can only be supported in a guide of infinite transverse dimensions, the group velocity becomescand the rest mass zero.