Structure of theEF
Structure of the electromagnetic field
The electromagnetic field may be viewed in two distinct ways: a continuous structure or a discrete structure.
Continuous structure
Classically, electric and magnetic fields are thought of as being produced by smooth motions of charged objects. For example, oscillating charges produce electric and magnetic fields that may be viewed in a 'smooth', continuous, wavelike fashion. In this case, energy is viewed as being transferred continuously through the electromagnetic field between any two locations. For instance, the metal atoms in a radio transmitter appear to transfer energy continuously. This view is useful to a certain extent (radiation of low frequency), but problems are found at high frequencies (see ultraviolet catastrophe).
Discrete structure
The electromagnetic field may be thought of in a more 'coarse' way.
Experiments reveal that in some circumstances electromagnetic energy
transfer is better described as being carried in the form of packets
called quanta (in this case, photons) with a fixed frequency. Planck's relation links the energy 
of a photon to its frequency 
through the equation:
where 
is Planck's constant, named in honor of Max Planck, and is the frequency of the photon . Although modern quantum optics tells
us that there also is a semi-classical explanation of the photoelectric effect—the emission of electrons from metallic surfaces subjected to electromagnetic radiation—the
photon was historically (although not strictly necessarily) used to
explain certain observations. It is found that increasing the intensity
of the incident radiation (so long as one remains in the linear regime)
increases only the number of electrons ejected, and has almost no effect
on the energy distribution of their ejection. Only the frequency of the
radiation is relevant to the energy of the ejected electrons.
This quantum picture of the electromagnetic field (which treats it as analogous to harmonic oscillators) has proved very successful, giving rise to quantum electrodynamics, a quantum field theory describing the interaction of electromagnetic radiation with charged matter. It also gives rise to quantum optics, which is different from quantum electrodynamics in that the matter itself is modelled using quantum mechanics rather than quantum field theory.
