Plexiglas is technically known as a poly(methyl methacrylate), sometimes known as a PMMA. It can be found under a variety of trade names, including lucite and perspex. It is a thermoplastic formed by industrial polymerization of methyl methacrylate. Due to its high light transmissivity and ability to flex under impact, it is often used as a replacement for glass in windows or other portals. It has a density approximately half that of glass, meaning it can save weight as well.
There are disadvantages to replacing glass with PMMA. In particular, it is highly vulnerable to solvent action (unlike glass and other ceramics). For another, it becomes brittle under heavy mechanical loads, unlike glass which handles compression loads easily. It is not nearly as hard as glass, meaning it can be scratched with little effort. Although it is more resistant to environmental loading (temperature fluctuation, rain, etc.) than its close competitor polycarbonate, the scratching problem and its vulnerability to ultraviolet light, which degrades its bonds, means that the harder polycarbonate is usually used in applications where resistance to damage is important (CD and DVD discs, for example).
Plexiglas was discovered in Germany in the late 1920s and marketed in the early 1930s. It saw widespread use in the Second World War especially in aircraft of all types, due to its better impact resistance and lighter weight than glass. In this application, it was discovered to be useful in medical chores as well. During air combat, many aircraft suffered damage to their canopy or windscreen, and in many cases fragments of those were blown into the eyes of their pilots. It was swiftly noted by the medics that although these fragments were, indeed, causing a great deal of trauma, they were not causing inflammation of the eye through irritation, and hence many of the wounds were not causing blindness. This was extremely rare, as few substances (other than glass and ceramics) were known to be tolerated by the eyeball. This finding led directly to the surgical replacement of components of the eye (usually the lens or parts of the cornea) with plexiglas components, and eventually to the modern contact lens. Hard lenses are constructed out of essentially PMMA, and soft lenses are modified plastics which are hydrophilic but still based on the PMMA formula.