This file for education only.

1. Chapter 10
Optical Properties
Mr. Pem PHAKVISETH
pempvs@gmail.com
Department of Materials Science and Engineering
Souphanouvong University
Faculty of Engineering
1

2. 10.1 Light and the Electromagnetic Radiation
Optical = “operating in or employing in the visible part of the electromagnetic
spectrum” or “ relating to sight, especially in relation to the action of light”.
Light is the form of energy detected by eye, and at ordinary scales can be treated as a
wave . Light wave are part of the electromagnetic spectrum, ranging continuously from
very long radio waves to high energy cosmic rays.
2

3. 3
00
1

=c
All electromagnetic radiation traverses a vacuum at the same velocity, namely, 3×108m/s.
This velocity, c, is related to the electric permittivity of vacuum 𝜀0 and the magnetic
permeability of a vacuum 0 through
c v=
‒  : Wavelength in meters per cycle
‒  : Frequency in Hertz (cycles/sec)
• The Wavelength and Frequency of
EM waves are related thru c
• EM radiation has a Wave ↔ Particle Duality
• The Energy, E, of a Light Particle
( ) chE == h
‒ Where h : Planck’s Constant (6.63x10-34 J-s)
‒ h is the PHOTON Energy

4. 4

5. 5
Visible light
380 nm  ອິດ 450 nm
450 nm  ຄາມ  480 nm
480 nm  ຟ້າ  510 nm
510 nm  ຂຽວ  565 nm
565 nm  ເຫຼືອງ  580 nm
580 nm  ແສດ  620 nm
620 nm  ແດງ  750 nm

6. 6
10.2 Refraction
• When a ray of light passes from air to water its speed changes.
Due to this there is a change in direction of ray.
• The change of direction suffered by a ray of light as it passes
obliquely from one optical medium to another optical medium
with different optical densities is known as refraction.
• Whenever a ray of light is incident on a surface separating two
media, a small fraction of the light always gets reflected.
• Refraction can make an object appear to be in different
position.

7. 7
   
v
Or
MatlinLightofSpdVacuuminLightofSpd
c
n
n
=
=
▪Now the relations for v and c
1v =
• Where ε and µ are respectively the Permittivity
and Permeability of the Material
▪Now Recall
001 =c
▪ Thus n
rr
c
n 


===
00
v
▪ Most Materials are NOT magnetic → µr  1
• So
• Ex. Germanium
rn 
▪ When light changes from space to air to water, it slows down. This changes the path
it takes. That’s refraction.
▪ The refractive index is the ratio of the velocity of light in space to the velocity inside a
different material.
❑ Define the INDEX of REFRACTION, n
2
15.76n − 
•n = 3.91
16.0r 

8. 8
• The slowing of light in a Non-Vacuum Medium Results in Refraction, or Bending of
the light Path
▪ Light Refracts per Snell’s Law :
2211 sinsin  nn =
Incident
ray
normal
Angle of Incidence
Angle of Refraction
Refracted ray

9. 9
10.3 Absorption, Transmission, Reflection
• Consider EM Radiation with Intensity I0 (in W/m2)
Impinging on a Solid
• The EM-Solid interaction Alters the incident
Beam by 3 possible Phenomena
• The EM Beam can be
• Reflected
• Absorbed
• Transmitted

10. 10
▪ An Energy Balance on the Solid:
• E-in = E-reflected + E-absorbed + E-transmitted
TAR IIII ++=0
•Now Divide E-Balance equation by I0
TAR ++=1
•Where:
–R  REFLECTANCE (IR/I0)
–A  ABSORBANCE (IA/I0)
–T  TRANSMITTANCE (IT/I0)
0I
RI
TI
AI

11. 11
1.Transparent Materials 2.Translucent Materials 3.Opaque Materials
Classification of Optical Materials
•Opaque →
- T = 0
•Transparent →
– T >> A+R
– Light Not Scattered
• Translucent→
–T > A+R
–Light Scattered

12. 12
• Metals Interact with Light Thru QUANTIZED Photon Absorption by Electrons
▪ Metals have Very Closely
Spaced e- Energy Levels
Metals – Optical Absorption
•Thus Almost ALL incident
Photons are ABSORBED
within about 100 nm of the
surface
• Unfilled electron state are adjacent to filled states
• Near-surface electrons absorb visible light

13. 13
• The Absorbed Energy is ReEmitted by e- “falling” back to Lower Energy states
▪ Since Metals have Very
Closely Spaced e- Energy
Levels The Light is emitted
at many ’s
• Thus Outgoing Light Looks About the Same
as Incoming Light → High Reflectance
Metals – Optical Reflection

14. 14
• The reflectivity R represents the fraction of the
incident light that is reflected at the interface,
• If the light is normal (or perpendicular) to the
interface, then.
• When light is transmitted from a vacuum or air into a solid
s, then
Reflection
Optical Properties – Non-Metals

15. 15
• Example: For Diamond n = 2.41
• Reflection losses for lenses and other optical instruments are
minimized significantly by coating the reflecting surface with very
thin layers of dielectric materials such as magnesium fluoride
(MgF2).
𝑅 =
2.41 − 1
2.41 − 1
2
= 0.17
0.17% of light is reflected

16. 16
• In The Case of Materials with “Forbidden” Gaps in the Band Structure, Absorption Occurs only if h>Egap
incident photon
energy h
Energy of electron
filled states
unfilled states
Egap
Io
blue light: h = 3.1 eV
red light: h = 1.7 eV
Absorption
• The Material Color Depends on the
Width of the BandGap
▪For These
Materials there is Very
little ReEmission

17. 17
Color Cases – BandGap Matls
• Egap < 1.8 eV
• ALL Visible Light Absorbed; Solid Appears
Gray or Black in Color
• e.g., Si with Egap = 1.1 eV
• Egap > 3.1 eV
• NO Visible Light Absorbed; Solid Appears Clear and Transmissive
• e.g., Diamond Egap = 5.45 eV, SiO2 Egap = 8-9 eV
• 1.8 eV < Egap < 3.1 eV
• Some Light is absorbed and Material has a color

18. 18
NonMetal Colors
• Color determined by the sum of
frequencies
• transmitted light
• re-emitted light from electron
transitions
• e.g., Cadmium Sulfide (CdS)
• Egap = 2.4eV
• Absorbs higher energy visible light
(blue, violet),
▪ CdS
• Red/yellow/orange is
transmitted and gives it
this color

19. 19
Light Absorption/Reflection
• Amount of NON-Reflected Light Absorbed by a Materials
• For normally incident light passing into a solid having an
index of refraction n:
−
= e0IIT
 = absorption coefficient, cm-1
 = sample thickness, cm
= NonReflected incident light intensity
= transmitted light intensity
0I
TI

20. 20
Total Transmission
• Combining External and Internal Reflection, along with Beer’s Absorption
Yields the TOTAL Transmission Equation
( ) −
−= eRIIT
2
0 1
The transmission of light through a transparent medium for which there
is reflection at front and back faces, as well as absorption within the medium

21. 21
Total-T Example
• For the Situation at Right Determine the
thickness, d77, that will produce a total
Transmittance of 77%
• From Tab 21.1 Find Pyrex ns = 1.47
• Next find R using Equation (21.13)
13 mm
0I 086.0 I
QuartzPyrex
23 mm
%621.3
147.1
147.1
1
1
22
=






+
−
=





+
−
=
R
n
n
R
s
s

22. 22
• Recall total Transmission Eq
• Now Solve for β
13 mm
0I 086.0 I
QuartzPyrex
23 mm
( ) −
−= eRIIT
2
0 1
( )
2
0 1
TI
e
I R
−
=
−
( )
−=







−
2
0 1
ln
RI
IT
( )






−
−= 2
0
1
ln
R
IIT

• Thus
• Solving Total-T Eqn for the
length
• Then d77
( )
meter350.3
23
03621.01
86.0
ln 2
=






−
−=

 mm
( )






−
−= 2
0
1
ln
R
IIT

( )
mm0.56
mm
00335.0
03621.01
77.0
ln
77
277
=






−
−=
d
d

23. 23
10.4 Luminescence
• With luminescence, energy is absorbed as a consequence of electron excitations, which
is subsequently reemitted as visible light. When light is reemitted in less than a second
after excitation, the phenomenon is called fluorescence. For longer reemission times,
the term phosphorescence is used.
• Electroluminescence is the phenomenon whereby light is emitted as a result of
electron–hole recombination events that are induced in a forward-biased diode
• The device that experiences electroluminescence is the light-emitting diode (LED).

24. 24
• Based on EM Induced e− excitation, and then Relaxation with Broad-Spectrum h Emission
▪ e.g.
fluorescent
lamps

25. 25
10.5 Optical Fibers
• Fiber optic (or "optical fiber") refers to the medium and the technology associated
with the transmission of information as light impulses along with a glass or plastic
wire or fiber. Fiber optic wire carries much more information than conventional
copper wire.
• Most telephone company long-distance lines are now fiber optic.
• Optical fibers use light to send information through the optical medium.
• The light from an electromagnetic carrier wave that is modulated to carry information.

26. 26
Structure of Optical Fibers
• Glass Core – central tube of very thin size made
up of optically transparent dielectric medium and
carries the light form transmitter to receiver. The
core diameter can vary from about 5um to 100 um.
• Glass Cladding – outer optical material
surrounding the core having reflecting index lower
than core. It helps to keep the light within the core
throughout the phenomena of total internal
reflection.
• Plastic Covering– plastic coating that protects the
fiber made of silicon rubber. The typical diameter
of fiber after coating is 250-300 um.

27. 27
• When light (radiation) shines on a material, it may be:
-- reflected, absorbed and/or transmitted.
• Optical classification:
-- transparent, translucent, opaque
• Metals:
-- fine succession of energy states causes absorption and reflection.
• Non-Metals:
-- may have full (Egap < 1.8eV) , no (Egap > 3.1eV), or
partial absorption (1.8eV < Egap = 3.1eV).
-- color is determined by light wavelengths that are
transmitted or re-emitted from electron transitions.
-- color may be changed by adding impurities which
change the band gap magnitude (e.g., Ruby)
• Refraction:
-- speed of transmitted light varies among materials.
10.6 SUMMARY

28. 28
Thank you for your attention !!!
감사합니다 !!!
Department of Materials Science and Engineering
Souphanouvong University
Faculty of Engineering