0.14 Section 4.2. dielectric and its physics.

Section 4.2. Dielectric and its Physics.

Non-conducting materials are defined as dielectric. The surrounding sea of air is dielectric. Lenses, prisms and films are dielectric. Once light enters a dielectric we must consider μ =μ ₀ × μ _r and ε = ε ₀ × ε _{r .} That is relative permeability and relative permittivity must be accounted which in vacuum are unity.

Dielectric which are transparent in visible range are non-magnetic. Hence by the definition of Refractive Index n = c/v = √ε _r . This is Equation 1a in Section 4.1.

What does TRANSPARENCY mean in materials science ? Light can pass through a transparent material without absorption. This means light does not interact with transparent material. In scientific language it means that light does not interact with the medium it is passing through. If it interacts then the medium is not transparent.

Rewriting Eq 1a we get:

In Equation 5, Refractive Index ‘n’ is measured using visible light and the argument of the square root is Static Dielectric Constant which is quite different from High Frequency Dielectric Constant because of the basic physics which causes relative permittivity. Let us examine Water. The Refractive Index of Water is 1.333 but static dielectric constant of water is 80. So it is evident that refractive index is frequency dependent. This was evident some 300 years ago when Newton managed to disperse the Sun-light into seven colours of a Rain-bow. The fact that we can disperse light means that Refractive Index is frequency dependent.

4.2.1. Why is Refractive Index Frequency Dependent ?

In vacuum:

In a dielectric material:

In (7) and (8), D are the same and relative permittivity of Dielectric is greater than Unity hence the same Electric Flux creates a lower Electric Field in the medium as compared to that in the vacuum. So how is the remaining flux density accounted for. The remaining Flux density is causing Polarization and the scenario is the following:

The Application of an Electric Flux Density D in a dielectric medium causes an Electric Field as well as it induces Polarization.

In polar medium such as water, the dipoles get aligned along the line of Electric Field. This is defined as Orientational Polarization = P _{orientational} .

In non-polar medium, applied flux density distorts the electron cloud with respect to the nucleus of the atom as a result center of electron gets displaced with respect to the center of nucleus and a dipole is created temporarily. This is called Electronic Polarization = P _electronic .

In ionic crystals such as NaCl, the application of Electric Flux displaces positive and negative ions inducing dipole moments and producing Atomic Polarization or Ionic Polarization = P _atomic .

In Table 4.2. we give some examples of polar and non polar dielectric gases.

Table 4.2. Assorted Molecules and their dipole moments.