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1.1, 1.2 Lecture-1.pptx
1. Optical Properties of Solids
by
Mark Fox
Lecture 1
Prof. Dr. Safia Anjum
Chapter 1: Introduction
1.1 Classification of optical process
1.2 Optical coefficients
5. General phenomenon when light moves
from one medium to another:
Incident light
Propagation
through medium
Reflected light
Transmitted
light
(pass through
medium)
1st medium
(AIR)
2nd medium
(GLASS)
6. Refraction
“When the light rays either bend or change their direction while
passing from one medium to another it is called refraction of
light.”
Different phenomena of light
7. Cause of bending
For light, refraction follows Snell’s
law which states that;
Since the velocity is lower in the second medium (v2 < v1),
so ,θ2 < θ1 , that’s why the light bends .
air water
For a given pair of media, the ratio of
the sines of the angle of
incidence θ1 and angle of refraction
θ2 is equal to the ratio of velocities
(v1 / v2) in the two media.
9. Absorption
In a material, atoms vibrate at a specific frequency which is called
the natural frequency.
If a wave of light hits a material in which the atoms are vibrating at
the same frequency as the wave of light, the electrons will absorb the
light.
10. Due to absorption , objects have different colors.
When visible light strikes an object , a
specific Frequency which matches with
the frequency of atoms in material
becomes absorbed and the rest will be
reflected to our eyes
that will contribute to the color appearance
of that object.
Visible light consists of
seven colours, each has
specific frequency and
wavelength
12. Luminescence
This word Originates from latin word
luminare which means to light up or
illuminate.
Luminescence is spontaneous emission
of light by a substance not resulting from
heat.
oWhen a material absorbed energy from a source such as ultraviolet or X-
ray radiation , electron beams, chemical reactions,
and so on.
o This energy lifts the atoms of the material
into an excited state.
oBecause excited states are unstable, atoms are
back to unexcited ground state, and the absorbed
energy is liberated in the form of light .
13. Example
The firefly in the photo gives off light which is produced by a
chemical reaction by an organic compound in their abdomens.
14. Scattering
When a beam of light interacts with a particle of matter, it is
redirected in many different directions. This phenomenon is
called scattering of light
15. Example
When the sunlight strikes with the molecules
in our atmosphere like dust particles ,water
droplet, the light is redirected in many
directions . Sky is bright because the
molecules and particles scatters the light.
18. Definition
The different optical phenomena can be quantified by a
number of parameters which are called optical coefficient .
They determine the properties of medium, these optical
coefficients are;
Reflectivity (R)
Transmissivity (T)
Refractive index (n)
Absorption coefficient (α)
19. 1. Coefficient of reflection or reflectivity (R)
It is a parameter that describe the how much light wave is reflected by a
medium. It can be define as ratio of amplitude of incident wave to
amplitude of reflected wave
2. Coefficient of transmission or transmissivity (R)
It is a parameter that describe the how much light wave is transmitted
through a medium. It can be define as ratio of amplitude of incident wave
to amplitude of transmitted wave .
If there is only reflection and transmission of light by conservation of
energy we can say ;
the sum of the transmitted and reflected light is equal to the incident light
and it remains constant.
R+T=1
transmitted
Reflected
20. 3. Refractive index (n)
It is a parameter that describes how fast light travels
through the material. It is defined as
Where c is the speed of light in vacuum and v is the
velocity of light in the medium. For example, the
refractive index of water is 1.333.
4. Absorption Coefficient (α)
This parameter describes that how much light is absorbed by
medium. It is intensity attenuation of the light passing through a
medium.
21. Inciden
t light
x
Decrease in
intensity in
thickness dx
Decrease in intensity =light absorbed in region dx X
intensity at point x
intensity
Thickness
Graphically
22. As we know ; R+T=1
For the transmissivity;
T=1-R
But the reflection is from both front and back surfaces R1 and R2 ;
T=(1-R1)(1-R2)
Also there is some absorption in b/w two reflections;
T=(1-R1)exp(-αx)(1-R2)…….(1)
Term exp(-αx) gives the decrease in intensity .
Now if front and back sufaces have equal reflections ;
R1=R2=R
Put in eq (1)
T=(1-R)2exp(-αx)
This is expression for
transmissivity in terms of
Reflection and absorption.
23. O.D = -log
O.D =
Io
l
I )
(
l
l
e
434
.
0
10
log
The absorption of an optical medium can also be described in terms of
optical density (O.D) and it is defined as ;
The optical density is directly related to the absorption coefficient as ;
24. Stokes shift in luminescence
Excited state
relaxation
emission
absorption
Ground state
The atoms jumps to an excited state
by absorption of a photon.
Then relaxes to an intermediate state,
before re-emitting a photon by
spontaneous emission as it falls to
ground state.
The photon emitted has smaller
energy than absorbed photon. This
reduction in photon energy is called
Stokes shift.
25. Scattering
Scattering causes attenuation of light beam. The intensity decreases
exponentially as it propagate into medium as;
I(z)=Ioexp(-Nσsz)
Where;
N=number of scattering centres per unit volume
σs=scattering cross section
The scattering is described as Rayleigh scattering if size of scattering
centre is very much smaller than the wavelength of light. In this case;
4
1
s
Rayleigh scattering
