In this chapter, we examine some of the principal attributes of electromagnetic radiation that feature in photons. At the outset, it is worth asserting that the photon concept owes its legitimacy to the quantum theory of light. As we shall see in later chapters, the photon is formally regarded as a quantum of excitation of any specific optical mode. In the literature, one can find occasional mentions of photons qualified by an adjective, such as dipole photons, electric photons, ballistic photons, etc.,* These should be regarded as potentially misleading because each alludes to a property that is more correctly associated with a particular kind of phenomenon. Even if the distinction between "real" and "virtual" photons represents one of the most widely used descriptors, it does not signify a clear-cut difference. We thus begin by listing the following photon properties, all of which can be involved in specific forms of optical interaction, with the exception of the first.
Properties of a Photon