The phenomenon of interference and diffraction prove wave nature of light. These effects do not tell us about the type of wave motion viz longitudinal or transverse.
You need two tourmaline crystals for this experiment.
Let A and B are two tourmaline crystals cut parallel to their crystallographic axis. Let ordinary light i.e., un-polarised light fall normally on one face of the crystal A. Observe the intensity of light coming out of A. On rotating the crystal A about the incident ray as axis, no change in the intensity is observed. The second crystal B is now placed with its axis parallel to that of A. Now, both crystals are rotated simultaneously so that their axes are always parallel to each other. No change is observed in the intensity of light coming out of B. Now, keeping A fixed, let crystal B is rotated. The intensity of light gradually decreases. When B is perpendicular to A, no light emerges from B. If it is further oriented, the intensity gradually increases and becomes maximum when the axes of A and B are again in parallel.
The variation in the intensity of the emergent light shows that light waves are transverse in nature. If they were longitudinal, no change in intensity of the emergent light would have been observed.
The phenomenon due to which the vibrations of light waves are confined (restricted) in a specific direction perpendicular to the direction of wave motion is called polarization.
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