How can you detect a polarized light?

Explanation

Take two tourmaline crystals `T_(1)` and `T_(2)` cut with their faces parallel to the axis of the crystal. A fine beam of light from the source S is passed through `T_(1)` and light is observed by placing eye behind `T_(2)`.

When the axis of `T_(1)` and `T_(2)` are parallel, maximum light is observed [Fig. (i)] but when the axis of `T_(1)` and `T_(2)` are crossed (perpendicular), then practically no light is observed. This phenomenon can only be explained if light waves are transverse. Light falling on `T_(1)` has transverse vibrations lying in all possible directions. `T_(1)` allows only vibrations which are parallel to the axis of `T_(1)` . If second crystal `T_(2)` is placed parallel to `T_(1)` as in first figure, then vibrations transmitted by `T_(1)` are also transmitted by `T_(2)`. If `T_(2)` is perpendicular to `T_(1)`, the vibrations from `T_(1)` are perpendicular to the axis of `T_(2)`. Hence `T_(2)` will not allow these vibrations to pass [Fig. (ii)].
Longitudinal vibrations can however pass through slits irrespective of their orientations. This is the reason as to why the sound waves cannot be polarised.
Due to the change of the intensity of polarised light as a result of relative orientation of their crystallographic axis, it is concluded that the light waves must be transverse in nature.