A circular polarizer does not simply pass or block circularly polarized light by itself. It works in two stages:

1. A linear polarizing layer first lets through only light vibrating in one direction.
2. A quarter-wave plate behind it then converts that linearly polarized light into circularly polarized light.

So the reflection-reducing effect comes from the linear polarizer. The “circular” part refers to the state of the light after it passes through the filter, not to what kind of light is selectively removed at the start.

This matters because many cameras use beam splitters and autofocus/metering systems that can be confused by linearly polarized light. A circular polarizer avoids that problem by converting the transmitted light to circular polarization.

In simple construction terms, a circular polarizer is basically a linear polarizer plus a quarter-wave plate mounted together. The rotating front ring lets you choose the orientation of the linear polarizer to control how strongly reflections and sky darkening are affected.