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
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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. Light Interactions With Solids
When light proceeds from one medium into another (e.g. from air into a solid substance), several things happen. Some of the light radiation may be transmitted through the medium, some will be absorbed, and some will be reflected at the interface between the two media.
The intensity I0 of the beam incident to the surface of the solid medium must equal the sum of the intensities of the transmitted, absorbed, and the reflected beams, denoted as IT , IA , and IR , respectively.

10. Reflected: IR Absorbed: IA Transmitted: IT Incident: I0
• Incident light is either reflected, absorbed, or transmitted:
Incident: I0
Absorbed: IA
Transmitted: IT
Reflected: IR

11. • Optical Classification of Materials:
Materials that are capable of transmitting light
with relatively little absorption and reflection are transparent.
2. Translucent materials are those through which
light is transmitted diffusely; that is, light is
scattered within the interior, to the degree that
objects are not clearly distinguishable when
viewed through a specimen of the material.
3. Materials that are impervious / impenetrable to the transmission of visible light are termed opaque.

12. dense polycrystalline Porous polycrystalline
single crystal
dense polycrystalline
Porous polycrystalline
Transparent
Translucent
Opaque

13. Refraction of Light
When photons are transmitted through a transparent material, they loose some energy and speed and the direction changes.
Refractive Index =
Examples:
Silica glass
Diamond
C (Velocity of light in vacuum)
V (velocity of light in a medium)
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14. Generally, the larger an atom or ion, the slower the velocity of light, and the greater the index of refraction.
The index of refraction for a typical soda-lime glass is approximately 1.5. Additions of large barium and lead ions (as BaO and PbO) to a glass will increase n significantly. For example, highly leaded glasses containing 90 wt.% PbO have an index of refraction of approximately 2.1.
• Note: n = f ()
Typical glasses ca
Plastics
PbO (Litharge)
Diamond
air 1.00
polystyrene 1.59
--Adding large, heavy ions (e.g., lead
can decrease the speed of light.
--Light can be "bent"

15. Snell’s Law If light passes through one media to another
Total internal reflection if angle φ > φc
n = Refractive index of first media
n’ = refractive index of second media
φ = angle of incidence
Φ’ = angle of refraction
If light passes from media of high
refractive index to a media of
low refractive index, φ’ = 900
at φ = φc
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16. Optical Fibers Fiber components are the core, cladding, and coating.
The signal passes through the core, whereas the surrounding cladding constrains the light rays to travel within the core; the outer coating protects core and cladding from damage that might result from abrasion and external pressures.
Cross-section profile
Material: High purity silica glass
Diameter:5 – 100 μm

17. Optical Fiber Profiles
The index of refraction of cladding is lower than the core.
An optical fiber can transmit light for long distances because the light is continually reflected internally.
Fig , Callister 7e.

19. Absorption, Transmission and Reflection of Light
For a particular wavelength λ,
(Reflected fraction) λ +(Absorbed fraction) λ + (transmitted fraction) λ = 1
Metals:- Amount of energy absorbed depends on electronic structure.
Incident beam easily elevates electrons to higher levels.
Metals strongly reflect and/or absorb light.
Example: Gold absorbs shorter wavelength (blue & green) and reflects longer wavelengths (Yellow and red).
Silver and Al reflect all parts of visible light.
Metals absorb/reflect incident radiation from longest wavelength to the middle
of the ultraviolet range because the electromagnetic wave within this range are
sufficient to excite valence electron to the higher energy level.
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20. Silicate Glasses Reflection of light from a glass surface:
Fraction of light reflected = R =
R is called reflectivity (φi=900)
n = refractive index.
Absorption of light by glass plate: Light intensity decreases as light path decreases.
I = Fraction of light exiting
I0 = Fraction of light entering
α = linear absorption coefficient.
t = thickness
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21. Reflectivity, R
Example: Diamond

23. Transmittance Through a Glass Plate
Amount transmitted depends upon amount of light reflected from upper and lower surface and light absorbed.
Fraction on incident light reaching lower surface = (1-R) (I0e-αt)
Fraction of incident light reflected from lower surface = R(1-R)(I0e-αt)
I = [(1-R) (I0e-αt) ] – [R(1-R)(I0e-αt) ]
= (1-R)2 (I0e-αt)
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24. Plastics and Semiconductors
Plastics: Many plastics have excellent transparency.
If crystalline regions having high refractive index are larger than wavelength of light, the light will be scattered.
Semiconductors: In pure semiconductors, photons may cause electrons to jump across energy band gap.
The energy of photons should be greater than Eg.
Impure semiconductors absorb photons of lower energy.
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25. Superconducting Materials
Superconductivity: The resistivity of a metal drops suddenly to a immeasurable value upon cooling to critical temperature Tc.
For a material to be superconducting, materials critical temperature, magnetic field and current density must not be exceeded.
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26. Superconducting Materials

27. Magnetic Properties of Superconductors
If a critical magnetic field (Hc) is applied to a superconductor below Tc, superconductor returns to normal state.
Sufficiently high current density Jc will also destroy superconductivity.
Hc = critical magnetic
field at temperature T.
H0 = Critical field at 0 K.
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28. Type I and Type II Superconductors
Type I superconductor: Below Tc and Hc, applied magnetic field will be expelled from specimen except at the surface Meissner effect.
Type II superconductors: Magnetic flux is excluded from material up to a lower critical field Hc1.
From Hc1 to Hc2, field starts to penetrate and after Hc2, the behavior is normal.
Figure 14.26
Meissner effect
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29. Magnetic Properties

30. 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.

31. 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.
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32. 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.

33. 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

34. 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
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After C.S. Barrett, W. D. Nix, and A. S. Teteman, “Principles of Engineering Materials,” Prentice-Hall, 1973, p.459.

35. 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

36. 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).

37. 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.

38. 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
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39. 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.
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40. 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.
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42. 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
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43. Alignment of Magnetic Dipoles:
a)Ferromagnetism
b)Antiferromagnetism
c)Ferrimagnetism