As we saw in XX earlier chapter, if a light ray encounters a fluid or medium with a different index of refraction, the light bends towards or into the fluid with the higher index of refraction. This bending is what we call refraction. Let’s consider it a type of transmission. There are a number of flow vis techniques based on refraction. Let’s review Figure 1 shows a beam of light being transmitted through air before hitting a semi-circular glass lens. The majority (90%) of the beam is refracted, and passes on through the lens. About 10% is reflected from the glass surface.