When the light beam strikes the water stream, the laser light gets reflected. When the light travels from air into water, it slows down, causing it to change direction slightly. This change of direction is called refraction. When light enters a more dense substance, it bends more towards the normal line. Total internal reflection happens when a wave hits a medium boundary at an angle larger than a critical angle, with respect to the normal to the surface. When the light beam comes up through the water at an angle greater than about 49 degrees and total internal reflection takes place.