Optical device (4)
'optical device' is the definition.
(lens is a kind of optical device)
This is all the clue.
(Other definitions for lens that I've seen before include "Bender of beams" , "Camera component" , "Optical refractor" , "Aid for viewing" , "Photographer's accessory" .)
'optical device' is the definition.
(I know that prism is a type of optical device)
This is all the clue.
(Other definitions for prism that I've seen before include "Prims (anag)" , "Triangular shape of glass or quartz, breaks up a beam" , "Type of polyhedron" , "Light splitting optical device" , "Light-splitting object" .)