Light waves are electromagnetic waves and hence transverse in nature. The electric field in a light wave propagating in free space is perpendicular to the direction of propagation. But there are infinite number of directions perpendicular to the direction of propagation. but there are infinite number of directions perpendicular to the direction of propagation of light. for example if light propagates along x-axis, the electric field may be along y-axis or along the z-axis or along any direction in y-z plane in the ordinary light. However, if electric field `vecE` remains parallel to a fixed direction (say y-axis) then such light is called linearly polarised light.
There are several methods to produce polarised light from the unpolarised light. now-a-days polaroid sheets are commonly used to produce linearly polarised light. a polaroid has long chain of hydrocarbons which become conducting at optical frequencies. when light falls normally on the polaroid sheet, the `vecE` parallel to the chains is absorbed in setting up electric currents in the chains but `vecE` perpendicular to the chain gets transmitted. so, light on passing through the polaroid i.e., the transmitted light become linearly polarised with `vecE` parallel to the transission (pass) axis of polaroid.
If linearly polarised light of intensity 'I' is incident on another polaroid whose pass axis is inclined at an angle `theta` from pass axis of first polaroid (or transmission axis of `vecE` of linearly polarised light) the intensity of transmitted light `I_(t)` is given as:
`I_(t)=Icos^(2)theta`.
Q. What is the pass axis of a polaroid ?

Explanation

The pass axis of a polaroid is the direction in which `vecE` of light incident normally on polaroid sheet are allowed to be transmitted.