PMMA

PMMA or Poly(methyl methacrylate) is a transparent thermoplastic also called acrylic glass or perspex. wikipedia.

Chemically, it is the synthetic polymer of methyl methacrylate. It is sold under many trade names, including Policril, Plexiglas, Gavrieli, Vitroflex, Limacryl, R-Cast, Per-Clax, Perspex, Plazcryl, Acrylex, Acrylite, Acrylplast, Altuglas, Polycast, Oroglass, Optix and Lucite and is commonly called acrylic glass, simply acrylic, perspex or plexiglas. Acrylic, or acrylic fiber, can also refer to polymers or copolymers containing polyacrylonitrile.

Use for FDM

Nophead managed to print with it by setting the hot end temperature at 220C, and heated bed to 100C. It wouldn't stick in a cold environment.

This is probably very incomplete maybe incorrect info. Follow the link/do more research if you want to duplicate!

Manually working with it

Perspex can easily be sawed and filed with a metal saw. For accuracy in sawing, use a shallow sawing angle. With fast(electric) sawing/drilling care has to be taken that it doesn't go over the melting temperature.

However sawing it yourself can be time consuming, it may be a good oppertunity to look up and visit a nearby hackerspace.

It can be polished. http://www.ultimatehandyman.co.uk/ACRYLIC_FINISHING.htm Another method used sanding well and then applying heat to use melting as last polishing step.(cant find link)

If there are no defects and it is well-polished, presumably it can be used as lens/optics.(to some degree)