Why would you use a plastic prism?

Answer 1

I take it you mean "instead of glass / crystal" as opposed to why use a prism at all.

1. Cheaper - Depending on the material and specification, a simple prism can be made on an extrusion basis and simply sliced to length making for a very inexpensive product.

2. Lighter - many plastics are much less dense than glass - if the task can be handled with plastic and weight is an issue, glass would be a poor alternative.

3. Stability - although glass may have a higher survival rate (temperature), a prism that must be exposed for some reason may well be more vulnerable to chipping than plastic

4. Transmission / Reflection properties. Although most of the visible spectrum will pass well through all clear glass / plastics, as you begin to approach IR and most especially UV, the material have VERY different properties. UVB and UVC for example being blocked by most glass - hence UVC lamps use quartz rather than regular glass.

To give more information, it would help to know if there is a specific focus to your question - eg: application specific - in which case other factors may or may not be worth mentioning.