Imagine a ray of light that is inside water. When the light ray hits the surface of the water (assuming air is above) the ray that goes into the air will refract, bending away from the normal. Some of the light will also reflect off the underside of the surface. As the angle of incidence becomes greater, a larger percentage of the light is reflected rather than refracted. When the incident angle is large enough (called the critical angle), all of the light is reflected. This is called total internal reflection, or TIR for short.
The answer would be:
Retroreflectors