TEP 2.5.05- 05 Half shadow polarimeter,rotation of ... - Phywe

Related topics
Lippich polarizer, Malus’ law.
Half shadow polarimeter,rotation of polarisation
through an optically active medium
TEP
2.5.05-
05
Principle
Optically active substances cause very slight rotations of the light polarisation plan, which the method of
crossed polarisation filters is not strong enough to measure. With this method, the direction of polarisation
of the analyser is perpendicular to that of the polarizer. If an optically active substance is placed between
them, the polarisation direction of the analyser must be corrected by the corresponding angle of
rotation of the plane of polarisation in order to obtain an intensity minimum again. A stronger adjustment
possibility for the determination of the angle is given with the half shadow polarimeter, used in this experiment
to measure the angle of rotation of the plane of polarisation caused by glucose-water solutions of
different concentrations.
Equipment
1 Base plate with rubber feet 08700-00
1 Polarimeter tube 08650-00
1 HeNe laser, 1 mW* 08180-93
1 Support for direct vision 08726-00
1 Adjusting support 35 x 35
mm
08711-00
1 Surface mirror 30 x 30 mm 08711-01
7 Magnet foot 08710-00
1 Lens support 08723-00
1 Mounted lens, f = +20 mm 08018-01
2 Polarisation filter
Polarisation filter for
08730-00
1 half shadow polarimeter 08730-01
Fig 1: P2250505
prisms
1 Metallic screen 08062-00
1 D-Glucose 1000g 30237-25
1 Dest. Water 31246-81
*alternative:
1 HeNe Laser, 5 mW 08701-00
1 Power supply and switch 08702-93
for 5-mW Laser
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TEP
2.5.05-
05
Half shadow polarimeter,rotation of polarisation
through an optically active medium
Fig.2: Experimental set up for the demonstration of a half shadow polarimeter (* only required for 5 mW laser)
Set up and performance
- The experimental set up is shown in figs. 1 and 2. The recommended set up height (height of beam
path) should be 130 mm.
- To start with, cuvette K containing the liquid is not taken into account for the set up.
- To begin with, the laser beam is adjusted by means of adjustable mirror M in such a way that the
beam, widened by lens L, is cut in half by half shadow polarizer P2 (if the pointers of all polarizers
are set to zero, a well illuminated half circle appears on the metallic screen).
- The direction of the plane of polarisation of half shadow polarizer P2 is rotated by an angle ε = 10°
against that of polarizer P1 (pointer of P1 at zero, pointer of P2 at 80), as shown in fig. 2A. If the direction
of polarisation of analyser A is set perpendicularly to the direction of P1 (or respectively of P2)
(pointer of A set to 90 or 80), the corresponding half of the vision field appears dark on screen SC,
whereas the other half still is slightly lit (fig. 3b and c).
- If analyser A is rotated by the half angle ε/2 = 5° in its plane of polarisation (fig. 3d), both halves of
the vision field display the same luminous intensity (pointer of A = 85).
- The compensation of light intensity through the eye assures a very sensitive method to determine
angles. The sensitivity of the method increases with decreasing angles ∑ between polarizers P1 and
P2. On the other hand, the luminous intensity of the field of vision also decreases with the angle. An
angle ε = 5° or 10° is adequate for the following experiment.
- To determine the angle of rotation of the plane of polarisation through optically active substances, a
D-glucose/water solution of determined concentration C0, e.g. C0 = 0.24 g/cm3 is prepared in the
first place (that is, 24 g of glucose are weighed into a beaker glass and water is added until a total
volume of 100 ml is obtained).
- Glass cuvette K is completely filled with the sugar solution and brought into the beam path between
polarizers P1, P2 and analyser A (cf. fig. 2).
- If both halves of the field of vision on screen SC had been illuminated with the same intensity, one
now observes different luminous intensities. The polarising direction of analyser A must be rotated
by a certain angle α in order to obtain the same luminous intensity for both halves of the field of vision.
This angle α corresponds to the rotation of the plane of polarisation through the optically active
substance.
- It is now possible to determine the angles for different angles ε between polarizers P1 and P2; before
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introducing the optically active substance, analyser
A must be rotated by the half angle ε/2.
- This determination of angles is carried out for
sugar solutions of different concentrations. For
this purpose, the original solution c0 is diluted
with water to concentrations c0/2, c0/4 and
c0/8.
Half shadow polarimeter,rotation of polarisation
through an optically active medium
TEP
2.5.05-
05
Theory and evaluation
The function principle of the half shadow polarimeter is based on the fact that for an array of 2 polarizers,
of which the first acts as a polarizer and the second as an analyser, Malus’ law applies:
The first polarisation filter polarises light linearly,
and the analyser only is transparent for linearly
polarised light in a certain direction. If both polarisation
directions are directed the same way (φ =
0), intensity I after the analyser is equal to intensity
I0 before the analyser. In the case of crossed
polarizers, that is, if the direction of polarisation of
the analyser is perpendicular to that of the polarizer,
intensity I displays a minimum (φ = 90°).
Fig. 3: Working principle of the half shadow polarimeter
Fig. 4: Polarisation directions of the polarising filters: (a) without
optically active medium; the luminous intensity of
both halves of the field of vision are equal. (b)
Change of the direction of polarisation through an
optically active medium; the angel of rotation must
still be compensated by the same angle by rotating
the analyser, in order to obtain the same luminous intensities
on both halves of the field of vision
In the case of the half shadow polarimeter, on half
of the field of vision is covered by a polarizer rotated
by a small angle. Here φ is approximately
10°, so that according to (1), light intensity in this
half field decreases. In order to make use of the good distinction between different luminous intensities
by the eye, another polarizer is used as an analyser, which is nearly crossed against the two previous
polarizers (φ about 80°). According to (1), luminous intensity after the analyser is relatively weak and
generally different in both halves of the field of vision. Luminous intensities in both halves of the field become
equal when the direction of polarisation of the analyser corresponds to half the angle by which the
two previous polarizers were rotated (cf. again fig. 3).If the plane of polarisation is once more rotated by
a given angle between polarizer and analyser through an optically active medium, the analyser must be
rotated by the same angle in order to obtain the same intensity on both halves of the field of vision (cf.
fig. 4).
Optical activity is the property of some substances to rotate the plane of oscillation of linearly polarised
light propagating through them. This is caused by the fact that both components of a linearly polarised
beam travel with different velocities through this asymmetric medium, causing a phase shift between
them, which in turn causes a rotation of the plane of oscillation.
The specific rotation by a solution of an optically active substance is defined as the angle by which the
plane of polarisation of a light beam of the sodium D line (wavelength λ = 589.3 nm) is rotated at 20°C
throughout a length of 100 mm of the solution containing 1 g of the active substance per cm3. This is expressed
by the following symbol for D glucose:
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TEP
2.5.05-
05
Half shadow polarimeter,rotation of polarisation
through an optically active medium
If measurements are carried out sensibly at the same temperatures, the linear relation between angle of
rotation α and concentration c can be plotted on a graph. It must, however, be taken into account, that
the wavelength used in this case is λ = 633 nm.
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