The physics
In a transverse wave, the medium moves across the wave's path. A rope wave is transverse. A slot passes only the part of the swing along the slot.
Aafter = A cos θ
In a longitudinal wave, each part moves along the path. A slinky's push is longitudinal. Every slot lets it through. So polarisation needs a transverse wave.
Light is transverse too. Its field swings across the beam. A polaroid passes the part of the field along its axis. Unpolarised light loses half its brightness at the first polaroid.
I₁ = I₀/2
After that, the light's field lies along the axis. At the next polaroid, the brightness follows Malus's law. θ is the angle between the light's field and the new axis.
I₂ = I₁ cos²θ
Brightness goes as the field squared. So light takes cos²θ, and a rope's swing takes cos θ.