Light is a electromagnetic radiation. Ordinary light: It is unpolarised light. Polarised light: Vibration will be in only one plane. When light passes through polarizer ( quartz glass) then polarisation of light takes place. Polarisation of light: When unpolarised light transforms into polarised light then the process is known as polarisation of light. Optical axis: Axis which decides the plane in which light get polarised. Plane of vibration: Plane contains the direction of propogation of light and direction of vibration of light. From Fig.ABCD is the plane of vibration. ABCD perpendicular to EFGH. Plane of polarisation: This plane contains only direction of propogation of light. From Fig.EFGH is the plane of polarisation. Doubly refracting crystal: There are certain crystal by which two types of plane polarised light rays are obtained when unpolarised light incident upon it. Such type of crystals are known as