Rotation of a plane of fluctuations of polarized light. Saccharimetry.
Some crystals, solutions of many organic substances (sugar, an acid), and also some liquids find out property to rotate a plane of fluctuations of polarized light. Such substances are called optically active. The phenomenon consists in that, «that at passage through such substance of polarized light the plane of its fluctuations gradually rotates around of an axis of a light bunch on a corner proportional to thickness of the layer of substance passed by light. The phenomenon consists in that, «that at passage through such substance of polarized light the plane of its fluctuations gradually rotates around of an axis of a light bunch on a corner proportional to thickness of the layer of substance passed by light. Everyone optically active substance has two kinds: laevorotary and dextrorotatory (against and clockwise if to look towards to light), consisting of molecules which structure represents mirror display one to another.
For
solutions optically active substances the corner φ of rotation of a
plane of fluctuations monochromatic light depends upon nature
substance, temperatures, concentration C
and heights of a column of a solution through which passes light,
,
where α
- the factor named by specific
rotation.
Specific rotation is the corner of rotation increased in 100 time for
a column of a solution in height of 1 dm (10 sm) at concentration of
substance 1 г
on 100 ml of a solution, at temperature 200
C and at length of a wave of light
=
589
nm.
For
glucose
.
The corner
of rotation for the given substance depends on length of a wave of
light. Corner of rotation
approximately
inversely proportional to a square of length of a wave
of
light
-
the
constant, depending by nature of substances.
The method
of the research using the phenomenon of rotation of a plane of
fluctuations of polarized light, is called polarimetry.
For exception influence of a wavelength of light on a corner of
rotation,
polarimetry
is made in monochromatic light which probably to receive by means of
a corresponding optical filter. If in the considered conditions to
use polarized white light, then at turn the analyzer will serially
pass beams of various length of a wave and a spot on the screen will
change accordingly color. This phenomenon is called a dispersion
of optical activity
and is used at studying structure of substance.
This
method
is
called spectropolarimetry.
Polarimetry
is used for definition of concentration optically active substances
in a solution, and in medicine, in particular, for definition of the
maintenance of sugar in urine of patients. The device applied to it
is called a
saccharimeter.
To simplification of calculations a tube with a solution is applied.
Length of this tube should be such that the corner of turn of the
analyzer in degrees must be numerically equal to concentration of a
solution in grams on 100 ml.
,
for
example, for glucose l=19
sm.
The saccharimeter (fig. 7 a) represents tube closed by a cover on support, in which the optical system of the device is located. The optical system of the instrument is schematically shown on fig 7 b. The parallel bunch of monochromatic light formed from a source by means of a yellow optical filter and a lens, passes through a polarizer, a tube with an investigated solution and the analyzer fixed on a rotating disk with divisions. Then light is focused by an objective in sight an eyepiece which is observed by an eye. At measurements without an investigated solution, the analyzer is established on full blackout of a field of vision. Then a tube with a solution is placed in the device and, rotating the analyzer, again it is necessary to achieve full blackout of a field of vision. Least of two corners, on which it is necessary to turn the analyzer, is a corner of rotation for investigated substance. On size of a corner concentration of sugar in a solution is calculated.
D
efinition
of a corner of rotation on blackout of a field of vision is
insufficiently exact and demands preliminary adaptation of an eye. At
the same time the eye is rather sensitive to a difference of
brightnesses bordering parts of a field of vision.
It
is used in polarimeter with penumbral readout in which the analyzer
is established on a condition of equalizing of brightnesses of two or
three parts into which the field of vision is divided.
Polarized
light can be used in modelling conditions for an estimation of the
mechanical pressure, arising in bone tissues. This method is based on
the phenomenon of photoelasticity,
which consists in occurrence of
Fig. 8.
optical anisotropy in originally isotropic firm bodies under action of mechanical loadings.
The flat model of a bone is made of a transparent isotropic material, for example plexiglass. In crossed polaroid this model is imperceptible, because dark. Anisotropy of plexiglass is created by loading, it becomes appreciable on a characteristic picture of strips and spots (fig. 8). On this picture, and also on its change at increase or reduction of loading it is possible to do conclusions about the mechanical pressure arising in model, and, hence, and in the present bone.
Circular polarimeter is intended for measurement of a corner of rotation of a plane of polarization optically active transparent homogeneous solutions and liquids. Polarimeter it is applied in medicine, the food, chemical industry and other branches of a national economy.
Fig. 9.
Design of the device
The optical basic scheme
Fig. 10.
The optical basic scheme includes: a lamp 1, an optical filter 2, condenser 3, polarizer 4, a chromatic phase plate 5, protective glass 6, two cover glasses 7, tubes 8, 9, 10 and 11, the analyzer 12, an objective 13, an ocular 14 and two magnifiers 15.
РР -direction of a plane of polarization of a polarizer;
АА -direction of a plane of polarization of the analyzer;
RR -direction of an optical axis of a chromatic phase plate;
ВВ -direction of a plane of polarization of radiation after a chromatic phase plate