CRITICAL ANGLE

REFRACTION VIA SNELL'S LAW FOR WAVES GOING INTO A HIGHER VELOCITY MATERIAL CAN CAUSE

TOTAL INTERNAL REFLECTION

WHERE TRANSMITTED WAVE IS ALONG INTERFACE

IN THE EARTH, THIS CAUSES REFRACTED WAVES ALONG THE BASE OF THE CRUST / TOP OF MANTLE (MOHOROVICIC DISCONTINUITY)

OTHER APPLICATIONS INCLUDE FIBER OPTICS WHERE LIGHT IS "TRAPPED" IN A LOW VELOCITY FIBER