A light wave is said to be linearly polarised if its electric field vector vibrates just in one direction perpendicular to the direction of propagation. Only tranverse waves can be polarised. Light can be polarised by reflecting it from a transparent medium. The extent of polarisation depends on the angle of incident. At a particualr angle of incidence, called Brewster angle, the reflected light is completely polarised as shown below :
Numerical : `l=l_(0) cos^(2) theta=l_(0)xx((1)/(sqrt2))^(2)=(I_(0))/(2)`
Resultant intensity `l_(R)=l cos^(2) theta=l_(0)(I_(0))/(2)xx((1)/(sqrt2))^(2)=(I_(0))/(4)`