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the "slits" are aligned.
This principle can then be used to determine if a compound rotates the plane of polarized light
and the sign of that rotation. If a solution of the compound is placed between the two Nicol
prisms, the plane in which the light vibrates will be changed if the compound is optically active
and unchanged if the compound is not optically active. If the two prisms are aligned before the
solution is placed between them, light will pass through both prisms; when the solution of an
optically active compound is inserted between them, the light will dim or be completely
absorbed. If the second prism is rotated, the light will gradually brighten until the second prism
is aligned with the new plane of vibration. If the number of degrees of this rotation from the
original position and the direction of the rotation is noted, the specific rotation of the compound
can be calculated. Rotation in a clockwise direction is considered to be a positive rotation
(dextrorotatory, designated as + or d), while rotation in the counterclockwise is considered to be
a negative rotation (levorotatory, designated as - or l).
This is the basis of the instrument called a polarimeter (A drawing of one type of polarimeter,
called a Zeiss polarimeter is shown in Figure 1.). In this instrument (Figure 2), a source of
monochromatic light, usually a sodium lamp emitting light at the sodium D line (wavelength =
5893 angstroms) is placed behind a N...
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- Spring '07