Unpolarized light of intensity I passes through a single polaroid, the intensity of polarized light is approximately 

CONCEPT:

• Malus law: This law states that the intensity of the polarized light transmitted through the analyzer varies as the square of the cosine of the angle between the plane of transmission of the analyzer and the plane of the polarizer.

I = Io cos2 θ          

Where Io = Intensity of incoming light and I = Intensity light passing through Polaroid

​EXPLANATION:

Polaroid:

• A polaroid consists of long-chain molecules aligned in a particular direction.
• The electric vectors along the direction of the aligned molecules get absorbed.
• When an unpolarized light wave is an incident on a polaroid then the light wave gets linearly polarized. 
• This direction of the electric field vector is known as the pass-axis of the polaroid. 
• The intensity of unpolarized light is generally reduced to 50% of the original when passed through a single polaroid. Therefore option 3 is correct.