How does an unpolarized light incident on a Polaroid get polarized?

When an unpolarized light is passed through a polaroid, then the light wave gets linearly polarized with the electric field vector oscillating along a direction perpendicular to the aligned molecules (pass-axis of polaroid). The attached diagram shows polarization by reflection from a transparent medium. When light is reflected, it becomes linearly polarized with the electric field vector perpendicular to the plane of incidence.

After passing through polaroid A, intensity is given by:
I1 = I/2

Let polaroid C be placed at an angle θ to polaroid A. Then, by Malu's Law, output intensity from polaroid C is:
I2 = I1cos²θ

Angle between polaroid C and polaroid B is π/2 - θ.
I3 = I2cos²(π/2 - θ) = I2cos²θsin²θ
I3 = I/8sin2(2θ)

Given I3 = I/8
θ = π/4