In contrast to the string, the free electromagnetic field has not got an
extrinsic length parameter. However, the wave number provides an intrinsic length
parameter. Energy quanta, hω, emerge only for the transverse field components, for
which a potential energy analogously to that of the harmonic oscillator acts. The
analog to the longitudinal field modes is the free particle.
Keywords: Electromagnetic field, Energy quantum, Extrinsic length parameter,
Intrinsic length parameter, Longitudinal electromagnetic field, Quantization,
Transverse electromagnetic field.