We perceive the Universe primarily through electromagnetic radiation which exhibits dual characteristics: it behaves both as a particle (a photon) and as a wave. Let us first consider the wave nature of light which is the same for all electromagnetic radiation such as radio/TV waves, visual light, X-rays, cellphone transmissions etc. Although sound waves require a medium for their transmission, such as air or water, electromagnetic waves can propagate through the vacuum of space. A main feature of these waves is their wavelength, which is the distance between any two peaks of the wave (see Fig. 3.1) or equivalently, between any two identically placed points on the wave itself. The wavelength symbol is usually denoted by the Greek letter lambda, λ, which represents the Greek ‘el’ sound.