What are polaroids? How are they used to demonstrate that (i) light waves are transverse in nature, (ii) if an unpolarized light wave is incident then light waves will get linearly polarized?

Polaroid is a thin and large sheet made of crystalline polarizing material which is used to produces plane polarized beam of light. 

(i) Polaroids can be used to demonstrate the transverse nature of light waves.

• The electric field, associated with propagation of light waves is always perpendicular to the direction of propagation of the wave.
• If a light wave is incident on the polaroid, the electric vectors assosciated with the propagating wave, along the direction of the aligned molecules get absorbed. 

(ii) If an unpolarized light wave is incident on a polaroid then, the the light wave will get linearly polarised with the electric vector oscillating along a direction perpendicular to the aligned molecules.