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