**1. Introduction**

There are four fundamental forces in nature, namely:

(i) The strong nuclear force is the strongest among the four forces. The strong force is known to bind subatomic particles (such as protons and neutrons) to form nucleus.

(ii) The electromagnetic force which is in the order of 10<sup>−</sup><sup>2</sup> that of the strong force [1]. The electromagnetic force governs the interactions among electrically charged particles.

(iii) The weak nuclear force which is in the order of 10<sup>−</sup>14 of the strong force [1]. The weak force acts in each individual nucleons (i.e., collections of protons and neutrons) and is responsible for the radioactive decay when neutrons decay to protons and electrons.

(iv) The gravitational force which is the weakest among all forces. The gravitational force attracts any object with mass.

A field is a spatial distribution of quantity, which may or may not be a function of time [2]. To put it in simple terms, an electromagnetic field is basically the field produced as a consequence of positively and/or negatively charged particles, be at rest or in motion, and exerted forces among each other. The electromagnetic field consists of both the electric field and the magnetic field. During static condition, both electric and magnetic fields exist independently. When only an electric field is present and is constant in time, the field is known as an electrostatic field; similarly, when only a constant magnetic field is present, it is known as a magnetostatic field. When the fields change over time (i.e., in time-varying condition), however, both fields have to be concurrently present. This is to say that a time-varying electric field induces a time-varying magnetic field and vice versa [1], resulting in both fields being coupled together.

 Due to its particle-wave duality nature, an electromagnetic field can be viewed as a continuous field which propagates in a wavelike manner, while at the same time, it can also be seen as quantized particles called photons. When the wave of the electromagnetic field propagates in an isotropic homogeneous medium, the electric and magnetic field components are mutually transverse to the direction of the energy transfer, as depicted in **Figure 1**. The radiation is therefore known as a transverse electromagnetic or TEM wave.

**Figure 1.**  *Transverse electromagnetic wave propagation.* 

 The distance between two adjacent troughs or crests of the electromagnetic wave is known as a wavelength. The wavelength is inversely proportional to the frequency of the wave (i.e., the tendency in which the wave repeats the same wave pattern). In other words, if the wave tends to repeat its cycle at a faster pace, the wavelength will become shorter. Likewise, if the pace of repetition decreases, the wavelength will become longer. The relationship between wavelength λ and frequency *f* can be expressed as (1) below:

$$
\lambda = \frac{1}{f\sqrt{\mu\epsilon}} \tag{1}
$$

 \_\_\_ where 1 \_\_\_ is the velocity of the wave propagation. Here, *ε* and *μ* are, respectively, √ the permittivity and permeability of the medium where the wave propagates. The permittivity *ε* measures the degree a material is polarized by the electric field, whereas the permeability *μ* dictates its ability in supporting the development of the magnetic field. Both electric and magnetic flux densities are, therefore, in direct proportion with *ε* and *μ*, respectively. Together with the conductivity parameter, σ, which describes the ease at which a charge can move freely in a material, these three parameters (i.e., *ε*, *μ,* and σ) are referred to as the constitutive properties of the material.
