{HEAD ~ Refractive elements} generally cause {TAIL ~ the light} to change direction and include lenses that can be used to focus or collimate light 
generally cause
{ HEAD ~ Refractive elements } generally cause { TAIL ~ the light } to change direction and include lenses that can be used to focus or collimate light
HEAD HEAD HEAD HEAD HEAD HEAD REL REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH