Why does a text become blurred, looking at it through a polyethylene film?

If you cover printed text with a piece of transparent polyethylene film you can still easily read it. As you gradually lift up the film, the text becomes increasingly blurred and may even disappear. Why that?
I guess it has something to do with the structure of the film's surface, but I'm not sure. What do you think? You have any suggestions on how to solve this problem?

I'm glad about every help!
tim




PS: I'm sorry about my bad english...

 

[HallsofIvy]

It has to do with the fact that the film bend as you pick it up so that it becomes like a lens with varying focal lengths over its surface. You can imagine how confusing the image would become.

By the way, your English is excellent. I have no idea what your native language is, but if I wrote in it you wouldn't understand anything!

 

[The_oMeGa]

The film acts as a lens in that it refracts light. When the film is close, the amount of refraction is small because it has had little time to "spread out" the light waves. When it is farther away, it has had more distance to refract upon. You can see common examples of this by turning on a flashlight. The beams start out concentrated, and eventually spread out.

That is all in very simple terms.