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Exception to the Polarizer Rule?

July 14, 2009

In performing an impromptu experiment yesterday, I discovered something fascinating. A circular polarizing filter (commonly referred to as a C-PL) will act on the reflections shown in reflections.

Let me back up and explain. For those who don’t already know, light that reflects off just about any non-metallic object is polarized to some degree. A polarizing filter will let you screen out polarized light when it’s coming from certain angles, relative to the filter. The practical effect of this is that you can use a C-PL to cut down on reflections. For example, a C-PL will let you see through glass or water that would otherwise have a horrific glare on its surface.

The upshot is that it only works with non-metallic surfaces. So yes on cars (their surface is paint, not metal) but no on things like gold, silver, chrome and the like. Also no on mirrors, which have a silver backing.

Here was the experiment. Even though a C-PL won’t cut the reflection from a mirror, what about the reflection seen on the window of a car while looking in the mirror. Did you get that? I looked in a mirror and saw a car reflected in the mirror. On the car’s windshield was terrible sun glare. On a whim, I whipped out my C-PL and looked to see what it would do. Amazingly, although it did nothing to change the reflection coming off the mirror itself, it did cut the reflection from the car’s windows, which could only be seen by looking in the mirror.

Now this information may have been common knowledge to some, especially those who wear polarized sunglasses, but to me it was a revelation.

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