Question : What do you mean by specific rotation ?
Answer : The amount of rotation of the plane of polarization while a polarized light travels an unit distance through an optically active substance of unit concentration is called the specific rotation of the substance.It depends on temperature and the wave length of the polarized light.
Note : A substance like cane sugar for which the rotation of the plane of polarization is to the right is called dextrorotatory substance and like fruit sugar it is to the left is called leavo-rotatory substance.
Question: what is a polarimeter ? Discuss its construction and working principle.
Answer : An optical instrument with which the rotation of the plane of polarization can be measured is called a polarimeter.
Construction and Operation : It consists of two nicol prisms P and A placed on two co-axial brass tube separated by a distance.P act as a polarizer and A as an analyser.They are able to rotate about a common axis.A Lorentz half shade plate H and a glass tube T are placed between P and A.A parallel beam of monochromatic unpolarised light is allowed to fall on P.Then a plane polarized light emerges from P.It travels through H and the substance within T.Then it falls on A.The beam emerges from A and reaches a telescope E.At first P and A are placed perpendicularly so that the field of view becomes dark.The glass ntube is filled with a optically active substance.When it is placed between the two nicol prisms P and A then the field of view becomes bright.If A is rotated to ba certion angle the former darkness comes back.This is the rotation of the plane of polarization.
Question :How will you determine specific rotation of a sugar solution by a polarimeter ?
Answer : The amount of rotation of the plane of polarization while a polarized light travels an unit distance through an optically active substance of unit concentration is called the specific rotation.
We take a sugar solution in the glass tube T and measure the rotation .