3D glasses which bend light about 5-10 degrees - how do they work?

A child’s jigsaw puzzle came with two pairs of cheap cardboard-framed 3D glasses.

The glasses had thin plastic film in them, as lenses.

These thin flat plastic lenses bend about 99% of visible light by around 5-10 degrees. You can see a very faint image of the unbent scene.

This produces a weak 3d effect when parts of an image have a black halo.

What is this thin film than bends light?

Why are the black halos required?