Electromagnetic radiation and interaction of light with with solids

Optical properties of material

Optical property of a material refers to response of material for electromagnetic radiation particularly the visible region of spectrum. The region of visible spectrum is \(\lambda=3700 A^\circ to 7000 A^\circ\).

Electromagnetic radiation:

Electromagnetic radiation consists of variation of electric and magnetic field. Light is electromagnetic radiation or photon. In classical sense, electromagnetic radiation is consider to be wave like having electric and magnetic field components are perpendicular to each other. Both components are perpendicular to direction of propagation as in pointing vactor

$$S=\frac{E\times B}{\mu_\circ}$$(energy flow per unit area per second)

Electromagnetic radiation is transverse wave having speed \(3\times 10^8 ms^-1\). The visible light in the spectrum is in between ultraviolet and infrared region of spectrum. The electromagnetic radiation consist of gamma-rays, X-rays, uv-rays, visible, infrared, microwave and radio wave (TV wave). The speed of electromagnetic wave in free space is given by,

$$c=\frac{1}{\sqrt{\epsilon_\circ \mu_\circ}}$$

Energy of photon is given by

$$E=hf=h\frac{c}{\lambda}\dotsm(2)$$

h=plan’s constant

\(\lambda\)=wavelength

Energy is proportional to frequency and inversely proportional with wavelength.

Light interaction with solid:

When light is incident on one medium and it is transmitted to another medium several things happen.

1. Some of the light radiation may be transmitted (T).
2. Some of the light will be absorbed (A).
3. Some of the light will be reflected at the interface between two medium (R).

If \(I_\circ\) be the intensity of incident beam on the surface between the two media then from conservation of energy,

$$I=I_T+I_A+I_R\dotsm(1)$$

Dividing equation (1) by \(I_\circ\) we get,

$$\frac{I}{I_\circ}+\frac{I_T}{I_\circ}+\frac{I_R}{I_\circ}$$

$$1=T+A+R\dotsm(2)$$

T=\(\frac{I_T}{I_\circ}\)=ratio of transmitted light to incident light is known as transmitivity

A=\(\frac{I_A}{I_\circ}\)=absobtivity

\(R=\frac{I_R}{I_\circ}\)=reflectivity=ratio of reflected to incident

• For transparent material, the value of T is approximately equal to 1.
• For opaque material, the value of T (transmitivity) is approximately equal to 0.

Transparent:

Material that are capable of transmitting light with little absorption and reflection are called transparent material. One can look through them the object opposite to the material. For example: window glass

Translucent:

These materials through which light is transmitted diffusely i.e. light is scattered within the material in such a way that the object is not clearly visible, such material is known as translucent. For example: thin plastic bag

Opaque:

Material through which light is not transmitted are known as opaque.

Atomic and electronic interaction:

The optical phenomenon that occurs within solid materials involves interaction between the electromagnetic radiation and atoms ions or electron. Two of the most important of this interaction are

1. Electronic polarization
2. Electron energy transition

Electron polarization:

One component of electromagnetic wave is a rapidly fluctuating electric field and other component is fluctuating magnetic field for the visible region of frequency this fluctuating electric field increases with electron clouds surrounding each atom. This electric field shift the electron cloud relative to the nucleus of atom with each other change in direction of electric field component. This type of oscillation creates two consequence of this polarization.

1. Some part of radiation energy may be absorbed
2. Light waves are retarded in velocity as they pass through medium.

Electron transition:

The absorption and emission of electromagnetic radiation may involve electron transition from one energy state to another when photon is absorbed by atom, the atoms gain energy of the photons and one of the atoms electron may jump to higher energy level, the atom is excited. When an electron of an excited atom fails to lower energy levels the atoms may emit excess energy in the form of photon. This phenomenon is known as electron energy transition. The exchange of energy between atom and radiation is given by,

$$\delta E=hf\dotsm(1)$$where

h=plank’s constant

f=frequency of radiation absorbed or emitted

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References:

Callister, W.D and D.G Rethwisch. Material Science and Engineering. 2nd. New Delhi: Wiley India, 2014.

Lindsay, S.M. Introduction of Nanoscience . New York : Oxford University Press, 2010.

Patton, W.J. Materials in industry . New Delhi : Prentice hall of India, 1975.

Poole, C.P. and F.J. Owens. Introduction To Nanotechnology. New Delhi: Wiley India , 2006.

Raghavan, V. Material Science and Engineering. 4th . New Delhi: Pretence-Hall of India, 2003.

Tiley, R.J.D. Understanding solids: The science of Materials. Engalnd : John wiley & Sons , 2004.