1. Optical Properties

2. Introduction
By “optical property” is meant a material’s response to exposure to electromagnetic radiation and, in particular, to visible light.
In classical sense, electromagnetic radiation is considered to be wave-like, consisting of electric and magnetic field components that are perpendicular to each other and also to the direction of propagation.

3. Spectrum of the electromagnetic radiation

4. Light and Electromagnetic Spectrum
Visible light: Electromagnetic radiation with wavelength 0.4 to 0.75 micrometers.
Ultraviolet : 0.01 – 0.4 micrometers
Infrared: 0.75 – 1000 micrometers
Light is in form of waves and consist of particles called photons.
ΔE = hν = hC/λ
ΔE = Energy
λ = wavelength
ν = frequency
C = speed of light
= 3 x108 m/s
H = plank’s constant
= 6.62 x J.s
15-2

5. Electron Transitions Δ E = hν
The absorption and emission of electromagnetic radiation may involve electron transitions from one energy state to another.
An electron may be excited from an occupied state at energy E2 to a vacant and higher-lying one, denoted E4, by the absorption of a photon of energy. The change in energy experienced by the electron, ΔE, depends on the radiation frequency as follows:
Δ E = hν

6. Important concepts:
Since the energy states for the atom are discrete, only specific ΔE exist between the energy levels; thus, only photons of frequencies corresponding to the possible ΔE for the atom can be absorbed by electron transitions.
A stimulated electron cannot remain in an excited state indefinitely; after a short time, it falls or decays back into its ground state, or unexcited level, with a reemission of electromagnetic radiation.

7. The energy difference =3.54eV-1.38eV= 2.16eV
A photon in a ZnS semiconductor drops from an impurity energy level at 1.38eV below its conduction band to its valence band. If ZnS has an energy band gap of 3.54eV, what is the wavelength of the radiation given off by the photon? What is the color of the radiation?
The energy difference =3.54eV-1.38eV= 2.16eV
=hc/E
=574.7nm
Visible yellow region

8. Wavelength vs. Band Gap Example: What is the minimum wavelength
absorbed by Ge?
(Given Eg = 0.67 eV, h = 6.62 x J.s, c = 3.0 x 108 m/s)
Answer:

9. Magnetic Properties

10. Introduction
Magnetism – a phenomenon by which materials assert an attractive or repulsive force or influence on other materials, has been known for thousands of years.
Many of our modern technological devices rely on magnetism and magnetic materials; these include electrical power generators and transformers, electric motors, radio, television, telephones, computers, and components of sound and video reproduction systems.
Iron, some steels and the naturally occurring mineral are well-known examples of materials that exhibit magnetic properties.

11. Magnetic Materials Very important in electrical engineering
Soft magnetic materials: Materials that can be easily magnetized and demagnetized.
Applications: Transformer cores, stator and rotor materials.
Hard magnetic materials: Cannot be easily demagnetized (permanent magnets).
Applications: Loud speakers, telephone receivers.
16-2

12. Basic Concepts Magnetic Dipoles
• Magnetic forces are generated by moving electrically charged particles.
• Magnetic dipoles are found to exist in magnetic materials, which, in some respects, are analogous to electric dipoles.
• Magnetic dipoles are influenced by magnetic field in a manner similar to the way in which electric dipoles are affected by electric fields.

13. Applied Magnetic Field
• Created by current through a coil:
Applied
magnetic field H
current I
N = total number of turns
L = length of each turn

14. Magnetic Fields
Ferromagnetic materials: Iron, cobalt and nickel -provide strong magnetic field when magnetized.
Magnetism is dipolar up to atomic level.
Magnetic fields are also produced by current carrying conductors.
Magnetic field of a solenoid is
H = 0.4П n i / l SI unit: A/m
n = number of turns
l = length
i = current
16-3
After C.S. Barrett, W. D. Nix, and A. S. Teteman, “Principles of Engineering Materials,” Prentice-Hall, 1973, p.459.

15. Response to a Magnetic Field
The magnetic induction, or magnetic flux density, denoted by B, represents the magnitude of the internal field strength within a substance that is subjected to an H field.
• Magnetic induction results in the material
current I
B = Magnetic Induction
inside the material
Unit for B: tesla or Wb/m2

16. Magnetic flux density in a material – dependence on permeability and magnetic field strength
The magnetic field strength and flux density are related according to
B = μH
The parameter μ is called the permeability, which is a property of the specific medium through which the H field passes and in which B is measured.
The permeability has dimensions of webers per ampere-meter (Wb/A-m) or henries per meter (H/m) or Tesla.meters per ampere (T.m/A).

17. Magnetic Induction
If demagnetized iron bar is placed inside a solenoid, the magnetic field outside solenoid increases.
The magnetic field due to the bar adds to that of solenoid - Magnetic induction (B) .
Intensity of Magnetization (M) : Induced magnetic moment per unit volume
B = μ0H + μ0 M = μ0(H+M)
μ0 = permeability of free space
= 4π x 10-7 (Tm/A)
In most cases μ0M > μ0 H
Therefore B M
Figure 15.3b
16-4
After C.S. Barrett, W. D. Nix, and A. S. Teteman, “Principles of Engineering Materials,” Prentice-Hall, 1973, p.459.

18. Magnetic Permeability
Magnetic permeability = μ = B/H
Magnetic susceptibility = Xm = M/H
For vacuum μ = μ0 = = 4π x 10-7 (Tm/A)
Relative permeability = μr = μ/ μ0
B = μ0 μr H
Relative permeability is
measure of induced magnetic field.
Magnetic materials that
are easily magnetized
have high magnetic
permeability.
Figure 15.4
16-5

19. Types of Magnetism
Magnetic fields and forces are due to intrinsic spin of electrons.
Diamagnetism: External magnetic field unbalances orbiting electrons causing dipoles that appose applied field.
very small negative magnetic susceptibility.
Paramagnetism: Materials exhibit small positive magnetic susceptibility.
Paramagnetic effect disappears when the applied magnetic field is removed.
Produced by alignment of individual dipole moments of atoms or molecules.
16-6

20. Ferromagnetism
Ferromagnetic elements (Fe, Co, Ni and Gd) produce large magnetic fields.
It is due to spin of the 3d electrons of adjacent atoms aligning in parallel directions in microscopic domains by spontaneous magnetization. Unpaired inner 3d electrons are responsible for ferromagnetism.
Random orientation of domains results in no net magnetization.
The ratio of atomic
spacing to diameter
of 3d orbit must be
1.4 to 2.7.
Parallel alignment of
magnetic dipole due to
positive exchange energy, e.g. Fe, Co, Ni.
16-7

22. Magnetic Moments of a Single Unpaired Electron
Each electron spinning about its own axis has dipole moment μB
μB = e h / 4 π m
In paired electrons
positive and negative
moments cancel.
Antiferromagnetism: In presence of magnetic field, magnetic dipoles align in opposite directions .
Examples:- Manganese and Chromium.
Ferrimagnetism: Ions of ceramics have different magnitudes of magnetic moments and are aligned in antiparallel manner creating net magnetic moments. E.g. Fe3O4.
μB = Bohr magneton
Unit SI μB=9.27x10-24 A.m2
e = electron charge
h = plank’s constant
m = electron mass
16-8

23. Alignment of Magnetic Dipoles:
a)Ferromagnetism
b)Antiferromagnetism
c)Ferrimagnetism