Download Polarization Experiment

Biophysics Department, Faculty of Science, Cairo University
[5]
Polarization Experiment
Aim:
Determination of the specific rotation of polarized light by sugar solution
Theory:
Polarized light:
Light is an electromagnetic wave at which the electric field E and the magnetic field B are
perpendicular to each other. Both fields vibrate in direction perpendicular to the direction of
propagation of light. Light waves may be polarized or unpolarized. Figure (1a) shows a
representation of a wave propagated at x-direction, while the electric field vector E in it
vibrates in the y-direction as represented in Figure (1b). The plane which includes the E and
the beam direction o-o is called the plane of polarization (the plane of the paper). Light from
natural sources is unpolarized since E changes its orientation every 10-8 second owing to the
nature of the light emission from the sources as shown in Figure (1c).
Figure (1)
Polarizers:
Polarizers are devices transforms unpolarized into polarized light are called "polarizer". One
type of these polarizers which use the phenomenon of double refraction in calcite crystals is
called Nicole prism and is illustrated in Figure (2).
If unpolarized light passes through this prism from one side, the prism will allow for only the
component of E parallel to its principal plane to pass and all other components are obscured.
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Second Level, Practical Biophysics
Biophysics Department, Faculty of Science, Cairo University
Figure (2)
If a plane polarized light is incident on the Nicole prism so that there is an angle θ between
the plane of polarization and the principal section, thus only the component parallel to the
principal section will pass with intensity:
I = Io cos2 θ
(1)
If θ = 90o then I = 0 and the beam is totally obscured.
Optical activity:
The solution of some organic materials (like sugar) in water has the property of rotating
the plane of polarization of light through an angle θ about the beam axis (O-O) in
Figure (1 a and b). The value of θ increases as the concentration of the solved material (C)
increases and as the length L of the path traveled by the wave in the solution increases. This
phenomenon is called "Optical Activity".
Thus,
θ=SLC
(2)
where S is a constant called the specific rotation of the material.
Different optically active materials have different values of S.
The polarimeter:
The polarimeter is an instrument for measuring the value of S for different optically active
materials in the form of solution in water (or in any other inactive solvents). The light beam
comes from Na-Lamp. The instrument is provided with an objective lens and eye piece. It
consists as shown in Figure (3) of 3 main parts:
Figure (3)
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Second Level, Practical Biophysics
Biophysics Department, Faculty of Science, Cairo University
1. The polarizer P:
It consists of 3 Nicole prisms assembled in one unit inside a casing, The principal section of
the three prisms is not coplanar. The side view of these sections 0, 1 and 2 are shown in
Figure (4a).
Figure (4)
Light transmitted through these three prisms is divided into three polarized beams each with a
plane of polarization parallel to the direction of principle section 0, 1, 2 of the corresponding
prism. The circular field of view in the eye piece is divided into 3 parts 0, 1, 2 (Figure 4b)
each corresponding to one of the three beams.
2. A 20 cm solution column:
In which the optically active liquid (sugar) is inserted into.
3. The analyzer A:
It consists of one Nicole prism which can be rotated about an axis collinear with the solution
column axis. The angle of rotation θ of this prism can be measured in degrees with the aid of
a degree scale and a vernier. There are two distinct positions for the analyzer A:
i.
If the principal section of A is parallel to that of (0); the field 0 will be brightened while
that of 1 and 2 becomes dark (Figure 5a).
Figure (5a)
ii. If the principal section of A is parallel to that of (1) and (2); the field 1 and 2 will be
brightened and that of 0 becomes dark (Figure 5b).
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Second Level, Practical Biophysics
Biophysics Department, Faculty of Science, Cairo University
Figure (5b)
iii. Between these two positions there is position at which the three fields are dark as in
Figure (5c).
Figure (5c)
Experiment and Requirements:
Requirements:
Polarimeter - Sodium Lamp - Sugar solution of known concentration - graduated flask distilled water - beakers.
Fill the tube with distilled water and then rotate the analyzer while looking through the eye
piece, until three fields of illumination as those illustrated in Figure (5a) are observed. Focus
if necessary the eye piece in order to sharpen the distinct image or the three fields. Then
slightly rotate the analyzer so that the three fields become as shown in Figure (5b). Between
the two positions a and b of the analyzer, there is a position at which the intensity of the
illumination of the three fields is equal as in Figure (5c).
Note: in the experiment you are required to get different concentration of sugar in water. Use
the relation C1V1 = C2V2 to get a new concentration C2 from known concentration C1,
knowing the volumes of the two concentrations V1 and V2.
Procedure:
1.
Determine the angle θo of the polarizer at previously established position using the
vernier scale; this angle is the zero angle of rotation corresponding to the distilled water.
2.
Fill the tube with a solution of known concentration C1 and find the corresponding
reading θ1 and correct the reading to be θ1 = θ' - θo
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Second Level, Practical Biophysics
Biophysics Department, Faculty of Science, Cairo University
3.
Repeat step 2 for different concentrations and in each case, find the corresponding
reading θx.
4.
Plot a relation between the angle of rotation θ and the corresponding concentration C.
5.
From the slope of the straight line find the specific rotation of sugar solution S using the
relation:
S= θ / CL = slope / L
(3)
where L is the length of the tube in centimeter (=20 cm).
Safety and Precautions:

After connecting the power source, turn on the sodium lamp for about 10 minutes, only
when the lamp gives out yellow sodium light, the observation can be made.

While pouring the solution to be measured into the test tube, the screws at both ends of
the test tube should not be screwed in too tightly (usually screw down the screws
conveniently until no water leak occurs) so as to prevent the protective plate from
producing strain which may cause illumination change of the viewing field and affect the
accuracy in measurement.

You should have the bulb part of the test tube to be upward while measuring, so that the
air bubbles can be stored in the bulb part and observation and measurements will not be
affected.

The time for continuously using the sodium lamp tube should not exceed 4 hours. When
the sodium lamp is being used for a long period of time, the electric fan must be used, or
the sodium lamp should be turned off for 10-15 minutes so as to get it cooled before
using it again.

After being used, the test tube should be rinsed and cleaned with water or distilled water,
and should be wiped cleaned and stored up.

Never use unclean and hard cloth or paper to wipe the optics, so as to prevent the
surfaces of the optics from being scared.
39
Second Level, Practical Biophysics
Biophysics Department, Faculty of Science, Cairo University
Results:
Concentration (C)
Angle (θ)
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Second Level, Practical Biophysics