Transparent polygonal solid (5)
I believe the answer is:
prism
(Other definitions for prism that I've seen before include "Light-refracting element" , "Triangular shape of glass or quartz, breaks up a beam" , "Transparent 3 sided object that seperates light into colours" , "Device that breaks light into colours of rainbow" , "Glass triangle that separates white light into the colour spectrum" .)