Download optically active substances.

Summary so far….
E y2

E0 y 2

1.
2

Ex2
E0 x 2
2
Ex
Ey
E0 x E0 y
cos  sin 2 
(o ~  E )d
  0,2 ,4 ,6 ,..... 2n
or
  0,  ,3 ,5 ,..... (2n  1)
The emergent ray will be linearly polarized light.

3 5

2.    2 , 2 , 2 ,..... (2n  1) 2
The emergent ray will be circularly polarized if  is 450 otherwise
elliptically polarized light.
Note: ε=+ve for RCP or REP and ε=-ve for LCP or LEP
RETARDERS
Optical devices which introduce a phase difference between e- and orays. These are in the form of plates of doubly refracting crystal cut
in such a way that optic axis is parallel to the refracting surfaces.
(  E ~ o )d 

4
i.e.  

2
Quarter wave plate, produces
linearly polarized light for incident
plane pol light if =0 0r 900 (b/w
optics axis and E-vector)
(  E ~ o )d 

Quarter wave plate,
circularly if =450
2
else elliptically polarized light
i.e.   
produces
HWP convert RCP or REP light to LCP or LEP and vice verse.
PRODUCTION OF POLARIZED LIGHT
1. Plane polarized light:
Un-polarized light
Plane polarized light
2. Circularly polarized light:
Un-polarized light
Plane polarized light
Vibration makes 450
angle with optic axis.
3. Elliptically polarized light:
QWP
Elliptically
polarized
Un-polarized light
Plane polarized light
Vibration makes angle other
than 450.
ANALYSIS OF POLARIZED LIGHT
1. Plane polarized light:
If intensity varies from maxima to minima with zero during
the full rotation of analyzer then light is plane polarized.
2. Circularly polarized light:
No variation in - It may be a un
intensity.
polarized or
- It may be a circularly
polarized light
If variation in intensity is like
plane polarized light original
light is circularly polarized.
QWP
Otherwise, original light is
un-polarized.
3. Elliptically polarized light:
Variation of intensity - It may be a partially
from a maximum to polarized or
minimum but not zero - It may be an elliptically
polarized light
If variation in intensity is like
plane polarized light original
light is circularly polarized.
QWP
Otherwise, original light is
partially-polarized.
Quest: What about superposition of two circularly polarized light (RCP and
LCP) beams with same amplitude and wavelength.
Any plane polarized light wave can be obtained as a superposition of a left circularly
polarized and a right circularly polarized light wave, whose amplitude is identical.
This is the base of Fresnel’s theory of optical rotation
Try it mathematically too
Optical activity
The phenomenon of rotation of the plane of vibration
is called rotatory polarization and this property of the
crystal (substance) is called optical activity or optical
rotation and substances which show this property are
called optically active substances.
optical activity : First time experimentally observed by Arago in 1811.
I=0
Observation:
In the absence of Quartz, I=0.
Along optic axis
Vo=Ve or μo = μe
Or no phase or path diff along the optics axis
Or no birefringence
optical activity : First time experimentally observed by Arago in 1811.
Two Crossed Nicol
 

Observation:
analyser
polarizer
I


Quartz plate
In the absence of Quartz, I=0.
In the presence of quartz, I is not zero
In quartz, when optic axis is perpendicular to refracting face, Plane pol. Light should
pass without any change no birefringence, so I should be zero after analyzer.
Presence of intensity shows the rotation of the plane pol light by the quartz material
Conclusion: Plane polarized light is rotated because of quartz
There are two types of optically active substances:
• Righthanded or dextro-rotatory:Sodium chlorate, cane sugar.
• Left handed or leavo rotatory:Fruit sugar, turpentine.
Note: Quartz is an optically active substance.
Calcite does not produce any rotation.
Biot’s law for optical rotation
 
 : angle of rotation of the plane of vibration for any given wavelength.
: length of the optically active medium traversed.
 In case of solution or vapours
  C, C: concentration of the solution or vapour
 The total rotation produced by a number of optically active substances is equal
to the algebric sum of the individual rotations.
  1   2  3  ....  i
i
The anticlockwise rotations are taken +ve ;
while the clockwise rotations are taken -ve.
Applications:
1. To find the percentage of optically active material present in the solution.
2. The amount of sugar present in blood of a diabetic patient determined by
measuring the angle of rotation of the plane of polarization.
Fresnel’s theory of optical rotation
Fresnel’s theory of optical rotation by an optically active
substance is based on the fact that any plane polarized light may
be considered as resultant of two circularly polarized vibrations
rotating in opposite direction with the same velocity or
frequency.
Fresnel’s theory of optical rotation
This explanation was based on the following
assumptions:
A plane polarized light falling on an optically active
medium along its optic axis splits up into two circularly
polarized vibrations of equal amplitudes and rotating
in
opposite
anticlockwise.
directions–one
clockwise
and
other
Fresnel’s theory of optical rotation
In an optically inactive substance these two circular
components travel with the same speed along the optic
axis. Hence at emergence they give rise to a plane
polarized light without any rotation of the plane of
polarization.
Fresnel’s theory of optical rotation
In an optically active crystal, like quartz , two circular components
travel with different speeds so that relative phase difference is
developed between them.
If vR>vL the substance is dextro-rotatory
And if vR< vL the substance is leavo-rotatory
Fresnel’s theory of optical rotation
On emergence from an optically active substance the two
circular vibrations recombine to give plane polarized light
whose plane of vibration has been rotated w.r.t that of
incident light through a certain angle depends on the phase
diff between the two vibrations.
“Fresnel Theory of Rotation”
(optic axes perpendicular to refracting face)
Plane polarized means resultant of R and L.
Note: We can prove it mathematically.
Specific rotation
The specific rotation of an optically active substance at a given
temperature for a given wavelength of light is defined as the
rotation (in degrees) produced by the path of one decimeter length
in a substance of unit density (concentration)
10
 
or   
(If is in cm)
C
C
The unit of specific rotation is deg.(decimeter)-1(gm/cc)-1
T

T
The molecular rotation is given by the product of the specific
rotation and molecular weight of the substance.
Polarimeters
A device designed for accurate measurement of angle
of rotation of plane of vibration of a plane polarized
light by an optically active medium is said to be a
polarimeter.
Two Types:
•Laurent's Half shade polarimeter
•Bi-quartz polarimeter
Laurent's Half shade polarimeter
Half shade device-H
Glass tube-T1
S
V1
N1
H
Two nicols- N1 AND N2
T1
N2
T
V2
Telescope-T
Nicol N2 can be rotated and its position can be noted on a circular
scale with verniers V1 and V2
Monochromatic light after passing through N1 become plane
polarized with its vibrations in principal plane of nicol.
Half shade device (H)
• It consists of a circular plate with one half made up of
quartz cut parallel to the optic axis which is parallel to YY’
and its thickness is such that it produces a path difference of
/2 or a phase difference of  between ordinary and extra
ordinary components (half wave plate).
Y
X’
Quartz
X
Glass
Half shade device (H)
• Other half is made up of glass, so that glass absorbs same
light as quartz plate does. Quartz is a half wave plate.
P
Principal plane of polarising
nicol make an angle  with optic
axis of quartz plate. Thus
direction of vibrations through
N1 is along OQ and on passing
through glass plate they will
remain along OQ.
Y
Q

X’
o
Quartz
o
Glass
Y’
On passing through quartz plate it breaks up into e ray
component with vibrations parallel to optic axis means
parallel to YY’ and o ray component with vibration along
XX’.
X
As they emerges out because
of phase change produced
between them these combine
to form linear resultant
vibrations along OP making
an angle  with YY’.
P
Y
Q
 
X’
X
Quartz
Glass
Y’
When principal plane of nicol N2 is equally inclined to
two plane polarized beams means from glass portion
and quartz portion Then two parts will appear equally
bright or equally dark (Optic axis of N2 is in YY’ or
XX’).
Laurent's Half shade polarimeter
Specific rotation: Half shade device (H)
Y
1. Tube is filled with pure water and
reading of analyzer is noted for equal

illumination
2. Suger sol. is filled in tube, suger sol
rotates the plane of vibration of both O
coming from quartz and glass equally
3. Note the reading of analyzer for equal
illumination again for diff concentration
4. Plot a graph b/w concentration and θ
SPECIFIC ROTATION
10
S 
lC
Where l is length of tube T1 in cms.
C
X
Summary: Half shade device (H)
Q
P
Y
1. We use monochromatic source.
2. Half portion is made up of quartz
and rest half is of glass. Thickness
 
is in accordance to HWP (means X’
o
no use of rotation property).
Quartz
Glass
3. HWP changes the plane of
polarization of incident light
(OQ) as OP. Because it creates
phase change of π in between e
Y’
ray and o ray) .
4. As a result through nicol N2, we can get two semi circles
with different intensities (along XX’) and with same
intensities (along YY’) too.
5. Sensitively detects the rotation of plane of polarization.
X
Bi-quartz Device


    L   R t
2 
Rotation 1 / 
This is much more sensitive and accurate then Half shade device
polarimeter. But having major drawback for color blindness person.
Summary: Bi-quartz Device
•White light source is used.
•Two semicircular quartz plates
(Right and Left handed) with Optic
axis perpendicular to crystal surface
Y’
(rotation effect only).
L
•Device is designed for yellow color.
•YY’ is tint of passes
x
R
R
Y
B
Y
B
YR
x’
•If Analyzer axes is perpendicular to YY’
then yellow color will be disappeared and we
 


  L   R t
can get resultant of Red and Blue color
2 
(Reddish violet color) as min intensity.
Rotation 1 / 
This is much more sensitive and accurate then Half shade device
polarimeter. But having major drawback for color blindness person.
Daily life uses:
Polarization effects in everyday life
Communication and radar applications
Biology
Geology
Chemistry
Astronomy
Materials science
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