Refraction device (5)
I believe the answer is:
prism
'refraction device' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for prism that I've seen before include "Light-diffracting glass" , "Glass triangle that separates white light into the colour spectrum" , "Glass triangle, separates light into colours" , "Transparent solid for dispersing light" , "Optical device used to invert an image" .)