Possibly the most convincing demonstration of witchcraft in the optics section are these big optical polarizing filters. Polarizing filters ("polarizers") polarize light, hence the name. Ordinary sunlight or classroom light is randomly oriented, so passing that light through a polarizer cuts out half the light and only lets through the light in a specific orientation. When a polarizer is placed in the way of polarized light, between 0-100% of the light will pass through the polarizer based on the orientation. If the light is polarized in the same direction as the polarizer, it will all go through. If the light is polarized perpendicular to the polarizer, all of the light will be stopped.
In this first image, the two polarizers are in the same orientation.
In this image, they are perpendicular to each other, so no light can pass through both.
So what happens if a third polarizer is put between the two polarizers that are perpendicular to each other? Again, it depends on the angle, but adding another polarizer between the two big ones can twist the light in such a way that it can pass through both (with some losses, of course).
No, that's not a hole but in a black sheet of paper. That's a little square polarizer standing on end right in front of the back polarizer.