Transcript Elliptically polarized light

Retarders
This is a class of optical devices which introduce a
phase difference between extra-ordinary and ordinary
rays. 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.
Optic axis
 Et
 ot
(E ~ O)t = /4
path difference between
Extraordinary and ordinary
Rays.
=
t
Quarter wave plate
• this is plate of double refracting crystal having
thickness t such that path difference between E-ray
and O-ray is /4.
(E ~ O)t = /4
• thus if a plane polarized light is incident normally
on the refracting face of the quarter wave plate
making angle  with the principal section, we can
divide this vibration in E-wave and O-wave. After
transmission through the crystal they will have a
difference /2 and resultant of these two vibration
will be elliptically polarized one.
Half wave plate
• When the thickness of crystal is such that path
difference between E-ray and O-ray is /2 or
phase difference is  then wave plate is known
as half wave plate.
• Half wave plate is used to produce linearly
polarized light.
(E ~ O)t = /2
For full wave plate: (E ~ O)t = 
Production of plane polarized light
polarizer
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analyser
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intensity’
maximum
•Plane polarized light emerging from one nicol, then it fall to
Other nicol. It passes through unaffected if principal section
Of two are parallel to each other.
• vibrations emerging out of polarizer are parallel to its
principal section are also parallel to the principal section of the
Second nicol (analyser).
polarizer
 

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polarizer
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
analyser
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intensity’
maximum
analyser
Intensity
zero
Production of circularly polarized light
Quarter wave plate
Analysing nicol
Polarising nicol
•Two nicols are at crossed position. If quarter plate is
at 450 light passing through it will be circularly
polarized. No variation of intensity with the rotation
Of nicol.
•If the angle of q wave plate be other than 450 then
emergent light is elliptically polarized.
Plane polarized light
Plane polarized
Complete extinction
Twice in one rotation
• pass the beam of polarized light through nicol prism.
Rotate the nicol about incident light as axis, if there is
complete extinction of light twice in each revolution then
incident light is plane polarized.
circular polarized light
No change in intensity
circular polarized
• pass the beam of polarized light through nicol prism.
Rotate the nicol about incident light as axis, if there
intensity of light emitted from nicol unchanged then light
is circularly polarized or unpolarized.
Elliptically polarized light
elliptically polarized
change in intensity
From max to min
• pass the beam of polarized light through nicol prism.
Rotate the nicol about incident light as axis, if there
intensity of light emitted from nicol changes from
maximum to minimum then light is elliptically polarized
or mixture of unpolarized and polarized light..
circular polarized light to plane polarized
Q wave plate
Circular
polarized
nicol
Complete extinction
Twice in one rotation
• pass the beam of circularly polarized light through a q.
wave plate, it will divide into o ray and e ray and on
emergence total phase difference will zero or , means it
is plane polarized light, then place a nicol and rotate it.
elliptically polarized light to plane polarized
Q wave plate
elliptically
polarized
nicol
Complete extinction
Twice in one rotation
• pass the beam of elliptically polarized light through a q.
wave plate, it will divide into o ray and e ray and a phase
difference /2 is produced and on emergence total phase
difference will zero or , means it is plane polarized
light, then place a nicol and rotate it.
Optical activity
when a beam of plane polarized light propagates
through certain substances or crystals, the plane of
polarization of the emergent beam is not the same as
that of the incident polarized beam but has been
rotated through a certain angle. This phenomenon of
rotation of the plane of vibration is called rotatory
polarization and this property of the crystal and
other substances is called optical activity or optical
rotation and substances which show this property are
called optically active substances.
Optical activity
polarizer
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analyser
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Quartz plate
A quartz plate with its refractive face perpendicular to the optic axis is
inserted between nicols. After analyser the field becomes illuminated.
As the polarized light is directed along the optic axis of quartz plane of
polarization turns.
• The amount of optical rotation depends upon the
thickness and density of the crystal or concentration
in case of solution, the temperature and the
wavelength of light used.
• There are two types of optically active substances:
• Righthanded or dextro-rotatory:- the substances
that rotate plane of vibration in the clockwise
direction as seen by the observer facing the
emergent light. Sodium chlorate,cane sugar.
• Left handed or leavo rotatory:- the substances
that rotate plane of vibration in the anticlockwise direction as seen by an observer facing
the emergent light. Fruit sugar, turpentine.
• It has been observed that calcite does not produce
any change in the plane of vibration of the plane
polarized light.
• The property of rotation of plane of vibration is
not only possessed by quartz, but also by all
organic compounds whose molecules are
asymmetric like sodium chlorate, sugar crystal
and solutions like turpentine, sugar solution,
quinine sulphate solutions etc.
Biot’s law for optical rotation
• For a particular wavelength of light the angle of rotation of
a plane of polarization is directly proportional to the length
(l) of the optically active medium traversed.
• In case of solution the angle of rotation for a given path
length is directly proportional to the concentration (c) of the
solution.
• The angle of rotation is inversely proportional to the square
of the wavelength for a given length of the optically active
substance.
• Optical rotation obeys the law of addition. The rotation
produced by a number of optically active substances is
equal to the algebraic sum of the indivudal rotations.
  1   2   3  ....
  i
i
• For a particular wavelength of light and at a
particular temperature the optical rotation is
proportional to lc
•
 lc
 = Slc
S is called the specific rotation. It depends upon the
nature of the substance.
Optically active substance solvent
Cane sugar
water
specific rotation
(dm)-1 (gm/cc)-1
+66.50
Applications of optical rotation
• To find the percentage of optically active
material present in the solution.
• The amount of sugar present in urine of a
diabetic patient determined by measuring
the angle of rotation of the plane of
polarization.