Optical Properties of Materials

OPTICAL PROPERTIES OF MATERIALS

When light is made to incident on any material, it interacts with the material and several process, such as transmission, absorption, dispersion, reflection etc., takes place. The process may vary from material to material based on the properties of the material.

The optical properties of the materials varies especially due to the interaction of electro-magnetic field of light with the charges present in the material (i.e.) when light is made to incident on a material, the electric vector of the light waves forces the charges in the material to have displacement from its initial position and hence creates a dipole. Since the electric vector is oscillatory it creates oscillatory dipoles. The dipoles produced (i.e.) polarisation depends on the number of available free electrons in the materials.

Delayed transmission: The oscillatory dipoles (i.e.) after excitation, when returns to ground state by reradiating the same frequency as that of its excitation, without any loss in energy, then it is called delayed transmission.

The delaying action reduces the effective velocity of light in the material and hence the refractive index [ratio between velocity of light in vacuum to the velocity of light in the material medium] is increased.

Note: For a medium which does not have any polarizable charges, there is no delay action, in that case refractive index 1. Example: for air n=1.

Absorption

When a material is exposed to light, the light will be absorbed by the material based on the number of free electrons available in the material. The absorption takes place in metals, insulators and semiconductors in which metals will have more free electrons and hence, absorption will be more in metals than insulators and semiconductors.

Dispersion

When light is passed through a material medium, the refractive index varies with respect to the wavelength of the incident light. (i.e.) If the wavelength is less, frequency is high and the refractive index is also high, in otherwords, high frequency radiation is deviated more due to more refractive index.

Dispersion formula

When atoms are exposed to light, then the atoms are displaced from its mean position, creating a dipole. Hence we can say the displaced electrons as dispersion electrons. If there are N atoms with different frequencies say ω1, ω 2.... ω_n then we can write the dispersion formula as

where n* → complex refractive index

e → charge of the electron

m → mass of the electron

ε₀ → permitivity of free space

fk → number of oscillating dipoles

g_k → g₁, g₂,……g_k (constants)

The dispersion formula gives the relation between refractive index, frequency of incident radiation (w) and the frequency of oscillating dipoles (ω1, ω 2.... ω_n). Since the refractive index has direct relationship with the frequency it is called dispersion formula.

Anamalous dispersion

Definition: The range of frequencies where the refractive index decreases with the increase in frequency is called anamalous dispersion.

The materials other than transparent materials will have anamalous dispersion and the dispersion formula is given by equation(1), where the refractive index is found to be a complex quantity.

Normal dispersion

Definition: The range of frequencies where the refractive index increases with the increase in frequency is called normal dispersion.

In the case of transparent substances (eg.) prism, the absorption is very less. The polarisation which depends on resistive constant (g_k) will also be less and the natural absorption frequency (ω_k) will also be less. Therefore the natural absorption frequency of electron is excluded. Thus the refractive index becomes a real function. Thus, the dispersion formula is given by

Reflection

The reflecting phenomena depends on the density of polarisation. When light is made to incident on glass materials, polarisation will not occur and hence there is no absorption. Thus the light is transmitted instead of reflection.

On the other hand, when light is made to incident on a dense gas of free electrons, it polarises the charges. If the polarised charges have not lost their energy by delivering the energy to lattice vibrations etc., then the oscillatory dipoles will Re-RADIATE the energy and is called reflection.

Thus if a surface contains high density of polarisable charges, a large portion of the light beam will be reflected. Thus we can say that metal will act as a perfect reflector. (e.g.) Silver (metal) coated in mirrors, metallic coating etc.

Reflection coefficient: According to electro-magnetic theory when light falls on a material a portion of incident light is reflected normally by the surface of refractive index 'n'.

Then, reflection coefficient