In this chapter we shall digress and take a first look at some of the elementary particles and their properties, knowledge of which will be useful in several places in the following chapters. We shall take a more detailed look at this subject in Chapter 14, when we consider the important question of the stability of the proton. Consider first the particle associated with light or electromagnetic waves. An alternative description of radiation exists in terms of particles called photons. It was realized at the turn of the century by Planck and later by others that radiation consists of discrete chunks of energy which are called photons. This is one of the consequences of the quantum theory, about which we will learn more later. Photons have most of the attributes of particles, and they can be considered as such. An ordinary light wave consists of billions of photons travelling all together but if we were to measure the energy of the wave very precisely we would find that it is a multiple of a definite quantity, which can be considered as the energy of a single photon. The energy of a photon is usually quite small so for most practical purposes the energy of an electromagnetic wave can have any value. However, the interaction of light or electromagnetic wave with an atom or atomic nucleus takes place one photon at a time. It is important to consider the photon picture when considering these interactions.