Study of light (6)
I believe the answer is:
optics
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">study of light</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this in another clue)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for optics that I've seen before include "Spirit measures" , "Scientific studies of light and vision, the eyes" , "Topics about eyes" , "Science of light" , "Organs of sight, eyes" .)