Investigation of Optical Activity of Transparent
Cellophane Films
SANTANU HAZRA AND A. GHORAI1
Department of Physics, Jhargram Raj College
Jhargram, West Midnapore 721507; West Bengal; India
1
Present address
Department of Physics, Chandernagore College,
Chandernagore 712136, Hooghly, West Bengal, India
e-mail: [email protected]
ABSTRACT
Variation of rotation of the plane of polarization of a plane-polarized
light with thickness of cellophane paper is investigated by a polarimeter
and white light source.
1. Introduction
Due to accidental breakage of the glass cover
of the polarimeter tube in front of analyzer
nicol at the time of filling the sugar solution
inside the tube, finding no other alternative,
fine cellophane film was used at the mouth of
the tube for the completion of the experiment.
It was observed that the plane of polarization
rotates through cellophane films. It is
interesting to study the optical activity of solid
substance like transparent cellophane film or
paper by a polarimeter tube where optical
activity of liquid solutions are generally
investigated.
Physics Education • September − October
Although, the theoretical background of
this experiment is not new yet we shall try to
make our experiment distinct. Emphasis is
given so that undergraduate students can think
of new experiments other than experiments of
prescribed syllabus and nurture with
instruments to increase their skill in handling
the sets of data with new and novel techniques.
If two nicols are kept crossed and a
monochromatic beam of light is sent through
them, no light emerges. This is because,
unpolarized light is plane polarized along the
principal section of the first nicol after passing
through it. If the principal section of the
second nicol is normal to that of the first, the
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second nicol will obstruct light. If we place a
quartz plate in between the two nicols, which
are crossed, some light will come out. This
light can be completely extinguished by
turning the second nicol at a little more from
the crossed position. This shows that the beam,
after coming out of the quartz plate, remains
plane polarized but the plane of polarization
has turned through to some extent. This
phenomenon of rotation of plane of
polarization is known as optical activity and
the substance is called optically active
substance. Not only some transparent solid
substances are optically active, but also, liquids
exhibit optical activity.1
Experiments for rotation of plane of
polarization need a polarizer and an analyzer
nicol, a biquadrate crystal and a polarimeter
tube. Biquadrate crystal consists of two
semicircular plates of quartz cut from right
handed and left handed samples. Thickness of
each sample plate is 0.375 cm so that yellow
color is rotated equally in opposite directions
through 90°. Thus for sodium light the
polarized light is turned by 90° by two
semicircular quartz and this will be
perpendicular to the principal section of the
analyzer. So it is completely quenched. For
white light, different wavelengths will have
different rotations. Only yellow light will have
90° rotations and will be completely obstructed
by the analyzer. The emergent light, in which
yellow light is missing, will produce a dimgray-violet tint in both halves which is called
sensitive tint or tint of passage. For slight
rotation from tint of passage one half appears
bluish and the other reddish with a marked line
separating the two.
2. Basic Principles and Theory
Rotation of plane of polarization of an optically
active substance depends on (i) the length of
the substance traversed by the beam, (ii)
density of the substance for solids or
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concentration for liquids, (iii) wavelength of
the incident light and (iv) temperature of the
substance.2 We shall consider first two cases
for cellophane films.
If θ be the total rotation of the plane of
polarization of a plane polarized beam, when it
travels a distance L of a solid substance of
density ρ, then θ varies with the product of L
and ρ. Hence
θ = sLρ
(1)
Here s is called the specific rotation of the
substance. Clearly θ – L graph will be a
straight-line passing through the origin.
3. Experimental
Polarimeter tube is washed and dried. It is
placed in the holder between the polarizer and
the analyzer and is illuminated with white light
source. The analyzer nicol with vernier is
rotated unto tint of passage position2. The
readings for two tints of passage positions are
noted. Clean and clear circularly cut cellophane
films with approximate cross section equal to
that of the mouth of the polarimeter tube are
prepared for the experiment. Thickness of n
such films is measured by a spherometer after
determining its least count. If the thickness of n
such films be t then average thickness of any
t
cellophane film will be d =
= 13.75 μm.
n
One circularly cut cellophane film is placed at
the mouth of the polarimeter tube near the
analyzer nicol and the tube is now placed in
between polarizer and analyzer nicol. Analyzer
nicol is now rotated for tint of passage position
and the readings for two such positions are
noted. Another circularly cut cellophane film is
placed adjacent to the first at the mouth. So
light has to travel twice the average thickness
d. Analyzer nicol is again rotated in the same
direction unto tint of passage position and the
readings for two such positions are noted.
Since the circular scale can measure angle up
Physics Education • September − October
insufficient. This process is repeated with
decreasing number of cellophane films. A
graph of rotation θ against the number of
cellophane films (with total thickness
L=8d=110μm) is plotted which is supposed to
represent a straight-line relation passing
through origin. This is shown in Figure 1.
Since the thickness of all the cellophane films
are not equal, so we have to take the average
thickness for which the observed experimental
points do not lie exactly on a straight-line. This
may be due to the surface and volume
inhomogeneity of the cellophane films.
to 360°, so two verniers are first numbered and
the analyzer nicol is rotated always in either
clockwise or anti-clockwise direction to get
angles greater than 360°. The process is
repeated for passage of light through more
number of films till the two distinct positions
of tint of passage are observed.3 Care should be
taken so that analyzer nicol is rotated
continuously in one direction and after one
complete rotation 360° is added to new reading
and so on. In this case a maximum eight
number of cellophane films can be placed at a
time after which intensity of light is
1000
θ−L curve
rotation in degree
800
600
400
200
0
0
1
2
3
4
5
6
7
8
9
no. of cellophane
Figure 1
Specific rotation times the density of the
cellophane films is determined from the
approximate straight-line plot (Figure 1) of θ
θ
and L(sρ =
= 7.384°μm−1). A circular main
L
scale cannot measure angle more than 360°. So
the analyzer nicol is rotated continuously in
one direction either clockwise or counterclockwise to get total rotation accurately
eliminating backlash error. Readings in air are
Physics Education • September − October
considered as zero and the rotation is
determined from the difference between the
readings with n cellophane film and the zero
readings noting the number of full rotations of
the circular scale. The experiment is repeated
with distilled water inside the polarimeter tube
and cellophane films at the mouth of the tube.
This shows no variation in the pattern and thus
distilled water is optically inactive.
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4. Conclusions
Students may be asked to model a new
experiment other than experiments of
prescribed syllabus and nurture with
instruments to increase their skill in handling
the sets of data with new and novel techniques.
2. B. Ghosh, Advanced Practical Physics Vol. I
and II, 2nd edition, Sreedhar Pub., Calcutta, 2nd
ed. 2005.
3. Santanu Hazra, Investigation of optical activity
of transparent cellophane films, the project work
under supervision of Dr. A. Ghorai submitted to
Vidyasagar University, West Medinipur, in
partial fulfillment of B. Sc. Part II Physics
Honours Examination 2001.
5. References
1. B. Ghosh and K. G. Majumdar, A Textbook on
Light, Sreedhar Pub. 5th ed. 2003.
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