Optical element (4)
I believe the answer is:
lens
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">optical element</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for lens that I've seen before include "French city" , "Piece of glass, usually curved" , "Transparent optical device" , "Optical element" , "Light-focusing device" .)