A polarimeter is a scientific instrument used to measure the angle of rotation caused by passing polarized light through an optically active substance. ... The amount by which the light is rotated is known as the angle of rotation. The angle of rotation is basically known as observed angle.
The basic operation principle of a polarimeter comprises the following: One generates light with an accurately prepared linear polarization state, usually by passage through a polarizer. That light is sent through the optically active sample, which somewhat rotates the polarization direction.