Polarization: The phenomenon due to which the vibrations of the lightwave are restricted in a particular single plane is called polarization light.
Transverse nature of light
Let us consider A and B are two similar tourmaline crystals, cut from parallel to their crystallographic axis. The two crystals are placed parallel to each other. S is the source of the light kept in front of crystal A.
When the light is passed from crystal A, the emergent beam will be slightly colored due to the natural color of the crystal. On rotating this crystal, no variation in the intensity of the emergent beam is observed. When the two crystals A and B are placed in such a way that their axis is parallel, the light passing from crystal A and B are the same i.e., they are plane-polarized as shown in figure 9a). When they are rotated parallelly, no change in light is observed on passing A and B.
When crystal A is fixed and B is slightly rotated, the emergent beam from B gradually fades and completely extinguishes when the axis of B is made perpendicular to the axis of crystal A as shown in figure (b). If crystal B is further rotated, the emergent beam is gradually reappeared until the axis of A and B becomes parallel.
This experiment concludes that the light wave is transverse in nature. If the light wave is longitudinal, no variation in light emergent from B would have occurred.