Light is an electromagnetic wave that vibrates. When light vibrates in all directions it is unpolarised light. A polarizer is used to make the light polarised, i.e., the wave vibrates in a single direction. If a molecule is optically active, it rotates the plane of polarised light. A polarimeter helps to calculate the specific rotation of a molecule. This cannot be calculated on paper but only by observing a molecule using a polarimeter.
If a molecule rotates the direction of incident light to the right, it is dextrorotatory (d, +). However, if it rotates the direction of the incident light to the left, it is laevorotatory (l, -). If a mixture contains both laevorotatory as well as dextrorotatory molecules in equal quantity, the mixture is called a racemic mixture (±).