[討論] 普化報告
Optical rotation or optical activity is the rotation of linearly polarized
light as it travels through certain materials. It occurs in solutions of
chiral molecules such as sucrose (sugar), solids with rotated crystal planes
such as quartz, and spin-polarized gases of atoms or molecules. It is used in
the sugar industry to measure syrup concentration, in optics to manipulate
polarization, in chemistry to characterize substances in solution, and is
being developed as a method to measure blood sugar concentration in diabetic
people.
History
The two asymmetric crystals forms, dextro and levorotatory, of tartaric
acid.The rotation of the orientation of linearly polarized light was first
observed in 1811 in quartz by French physicist François Jean Dominique
Arago. Around this same time, Jean Baptiste Biot also observed the effect in
liquids and gases of organic substances such as turpentine. In 1822, the
English astronomer Sir Joun F.W. Herschel discovered that different crystal
forms of quartz rotated the linear polarization in different directions.
Simple polarimeters have been used since this time to measure the
concentrations of simple sugars, such as glucose, in solution. In fact, one
name for glucose, dextrose, refers to the fact that it causes linearly
polarized light to rotate to the right or dexter side. Similarly, levulose,
more commonly known as fructose, causes the plane of polarization to rotate
to the left. Fructose is even more strongly levorotatory than glucose is
dextrorotatory. Invert sugar, formed by adding fructose to a solution of
glucose, gets its name from the fact that the conversion causes the direction
of rotation to "invert" from right to left.
In 1849, Louis Pasteur resolved a problem concerning the nature of tartaric
acid. A solution of this compound derived from living things (specifically,
wine lees) rotated the plane of polarization of light passing through it, but
tartaric acid derived by chemical synthesis had no such effect, even though
its reactions were identical and its elemental composition was the same.
Pasteur noticed that the crystals came in two asymmetric forms that were
mirror images of one another. Sorting the crystals by hand gave two forms of
the compound: solutions of one form rotated polarized light clockwise, while
the other form rotated light counterclockwise. An equal mix of the two had no
polarizing effect on light. Pasteur deduced the molecule in question was
asymmetric and could exist in two different forms that resemble one another
as would left- and right-hand gloves, and that the organic form of the
compound consisted purely of the one type.
In 1874, Jacobus Henricus van 't Hoff and Joseph Achille Le Bel independently
proposed that the phenomenon of optical activity could be explained by
assuming that the chemical bonds between carbon atoms and their neighbors
were directed towards the corners of a regular tetrahedron. This led to a
better understanding of the three-dimensional nature of molecules.
Theory
Optical activity is a type of birefringence. Any linear polarization of light
can be written as an equal combination of right-hand (RHC) and left-hand
circularly (LHC) polarized light:
where is the electric field of the light. The relative phase between the two
circular polarizations, 2θ0, sets the direction of the linear polarization
to θ0. In an optically active material the two circular polarizations
experience different refractive indices. The difference in the indices
quantifies the strength of the optical activity,
.
This difference is a characteristic of the material (for substances in
solution it is given as the specific rotation). After traveling through
length L of material the two polarizations pick up a relative phase of
,
where λ is the wavelength of the light (in vacuum). Consequently, the final
polarization is rotated to angle θ0 + Δθ.
Generally, the refractive index depends on the wavelength (see dispersion).
The variation in rotation with the wavelength of the light is called optical
rotatory dispersion (ORD). ORD spectra and circular dichroism spectra are
related through the Kramers–Kronig relations. Complete knowledge of one
spectrum allows the calculation of the other.
In summary, the degree of rotation depends on the color of the light (the
yellow sodium D line near 589 nm wavelength is commonly used for
measurements), the path length L and the properties of the material (e.g. Δn
or specific rotation and concentration).
Areas of use
Sucrose solution concentration measuring experiment, demonstrating optical
rotation.For a pure substance in solution, if the color and path length are
fixed and the specific rotation is known, the observed rotation can be used
to calculate the concentration. This usage makes a polarimeter a tool of
great importance to those who trade in or use sugar syrups in bulk.
In the presence of magnetic fields all molecules have optical activity. A
magnetic field aligned in the direction of light propagating through a
material will cause the rotation of the plane of linear polarization. This
Faraday effect is one of the first discoveries of the relationship between
light and electromagnetic effects.
Optical activity or rotation should not be confused with circularly polarized
light. Circularly polarized light is often presented as a linear polarization
rotating as the light propagates. However, in this picture the polarization
completely rotates in a length equal to the wavelength (roughly one
micrometre) and it can happen in vacuum. In contrast, optical activity only
occurs in a material and a complete rotation occurs in a length of
millimeters to meters, depending on the material.
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