Subject: What makes the plastic in front of a calculator screen change colors?

When you take apart a calculator in front of the screen there is a strip of plastic, which is darkened.   Even when the calculator is "on" you can't see anything on the screen unless that piece of plastic is in front of it.  When you reverse that plastic you see the digits in white on a black background.  But normally it's black digits on a white background.  What I want to know is why that piece of plastic does what it does.

Re: What makes the plastic in front of a calculator screen change colors?