As it has been already explained the light coming out of tourmaline crystal P has vibrations only in one direction parallel to the crystallographic axis. This type of light which attains one sidedness is called linearly polarised or plane polarised light. If the vibration is parallel to the plane of paper, it is represented by double head arrow while the vibrations are perpendicular to the plane of paper, it is represented by a dot. When an ordinary light is passed through a tourmaline crystal, then light is polarised and vibrations are confined to only one direction perpendicular to the direction of propagation of light. This type of light is called plane polarised light.