Thing which when you view at different angles has a different picture. Has a hard plastic cover with lines?

Hi, I am wondering what the plastic-sort cover on some things such as the harry potter deathly hallows part 1 dvd where if you view it at one angle, you see lord voldemort when you view it at a different angle you see harry potter. its like a motion thingy. you view it at angle x, you see picture x, you view at angle y, you see picture y. it is like plastic and has lines on it. a demonstration can be foudn here : http://www.youtube.com/watch?v=43WxwZifr8gThing which when you view at different angles has a different picture. Has a hard plastic cover with lines?
Basically, if you were to take that cover off and look at the printed image underneath, you would see the both images sliced up and layed out in every other line. Think of it like the resolution of your computer. Even lines showing one picture, and odd lines showing another.



The key to seeing only one image at a time is the covering. Those lines are cut at angles that basically turn the sheet of plastic into a series of prisms. Even though they are clear, a quality of prisms is that when you look through them, they reflect light like a mirror.



When you you view it at angle x, you see picture x, you view at angle y, you see picture y because on each angle, the light is travelling a different path and bouncing off a different side. Since the prisms are so small, your brain sees the whole image, just like on a TV screen (well, sort of)Thing which when you view at different angles has a different picture. Has a hard plastic cover with lines?
A hologram card?

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