ABSTRACT
Electromagnetic radiation consists of electromagnetic waves that propagate with their
electric and magnetic field vectors perpendicular to each other and to the direction of propa-
gation. An electromagnetic wave is characterized by its wavelength, l, and moves in a vacuum
at the speed of light, c. Therefore, the number of oscillations per unit time, the frequency n, is
given by nZc/l. Electromagnetic radiation is classified as ionising when it is capable of ionising
atoms and molecules. In this case, the interactions with matter are most suitably described by
considering the electromagnetic wave as a stream of particles, the photons. Each photon carries
energy hn where h is Planck’s constant.
