Re: 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.

Sounds like you are well on your way to figuring out a lot of things!

The details will depend on the type of display the calculator uses, but it is probably safe to assume it has a LCD (liquid crystal device) display. You can see a picture of how such a display works here:
Basic characteristics of the TN-LCD

It is likely that the type of display you are looking at is a "twisted nematic" LCD. Basically, the liquid crystal part of a "TN-LCD" can alternately rotate the polarization of incoming light (or not) depending on whether a voltage is applied. The mechanism for this can be seen in the diagram about half way down the page referenced above: when a voltage is present, all the liquid crystal molecules line up. When the voltage isn't present, they all form a 'twist' and rotate the light. The calculator display is probably a reflection display, and so a little different, but the idea is the same.

When you reverse that piece of plastic (the polarizer), you are changing the conditions of which polarization gets to the liquid crystal, and so the light parts will turn black and the black parts will turn white.

-Greg