Why do we use a Polarimeter?

Hint: A polarimeter is an instrument that is used to measure the angle of rotation when it is passing from polarised light. When the polarisation is passing to the sample tube, it starts exhibiting angular rotation toward the left and right.

Complete step by step solution:
A polarimeter is a device for finding the polarisation direction of light or we can say that it is used to find the angle of rotation when it is passing through the optically active fluid. The role of finding the rotation is to help us to find the concentration of the solutions.

A polarimeter consists of the polarised light source, a graduated circle to measure the angle of rotation, a sample tube, and a polarised light source. The figure of the polarimeter is shown below,

Image: Polarimeter

Working of the Polarimeter: As we can see in the figure, the monochromatic light is passing through the plane polarising plate which creates the polarised beam of light. This polarised beam of light is passing through a sample which contains ion the glass tube. When the beam is passed through the sample it will pass through the second plate known as the analyzer which helps to rotate the beam either via mutual rotation or automatic rotation. Which helps us to detect the concentration of the sample that is present.

Note : There are two main types of applications of the Polarimeter. One is quantitative and another is qualitative. In the quantitative application we can find out the concentration of the sample whereas in the quantitative application, it helps in chemistry to find out the D and L isomeric forms.