The light beam is repeatedly reflected, back and forth, between {HEAD ~ input and output reflective surfaces} on {TAIL ~ the wedge} 
on
The light beam is repeatedly reflected , back and forth , between { HEAD ~ input and output reflective surfaces } on { TAIL ~ the wedge }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL