English: This picture shows a test to determine if sunglasses are polarized or not.

1: Two polarized sunglass lenses with polarizations parallel to each other. Polarized light comes through.

2: Two polarized sunglass lenses with one of the lenses rotated to a 90° angle relative to the situation in picture 1. Almost no light comes through due to all of it being blocked. This is described by Malus' law.

3: One non-polarized sunglass lens. Non-polarized light comes through.

4: One non-polarized and one polarized sunglass lens rotated to a 90° angle relative to their normal orientation in the sunglass frames. Polarized light comes through.

Sunglasses (or at least one of the lenses) are not polarized if the near complete blockage of light seen in picture 2 is not observed when one of the lenses is removed and rotated in front of the other lens.

Sunglasses known to be polarized can also be used to test other sunglasses in a similar manner by rotating them in front of the sunglasses being tested.