Hint: In wave optics, Since light waves are transverse in nature which means the vibrating amplitudes of wave are perpendicular to the direction of propagation of wave so, in order to restrict the vibrations in a plane is the phenomenon known as polarisation of light.
Complete answer: Let us first understand the unpolarised light and polarised light in brief: When a light wave is travelling and its amplitudes are vibrating in more than one plane is known as unpolarised light. Whereas, when a light wave is travelling in space and the amplitude of waves are vibrating in a single plane it’s called a polarised light. This whole phenomenon of converting an unpolarised light into a polarised light is known as polarisation of light. In order to convert unpolarised light into polarised light, a material is used which is called polariser. When a polariser is placed in between the path of a travelling unpolarised light wave in a particular direction, the polariser restricts the direction of vibrations of amplitude of a unpolarised wave into a single plane and hence the emerging light wave from polariser is a polarised light wave. Hence, a polariser is used to convert an unpolarised light into a polarised light.
Note: In general, the most common types of polariser used are photographic filters, liquid crystal displays and sunglasses. In theory, if we need to test whether an unpolarised light wave is coming or polarised light we use an analyzer to determine the state of polarisation of a light wave.