View Index of Refraction - IOR
The Index of Refraction (IOR) is a value used to specify the way that light is scattered as it passes through a material. You are most familar with the “bent straw” effect when viewing a clear glass of water and a straw. That is because the light traveling through the glass of water is behaving differently than when it travels through air. Since we live on Earth, all IOR values assume that 1.0 is the value for traditional linear light behavior. And thus, air has an IOR of 1.0. Water has an IOR of 1.33. In the glass example, do you know how many different IORs would need to be considered, in order to accurately render the scene so that it matches reality?
Three: 1.00 for the air, 1.33 for the water in the glass, and 1.52 for the water glass. That will get you the ballpark result. Why just ballpark? Well, the straw would also have an IOR, as all materials in our universe have an IOR, even those that we may percieve as opaque. Even opaque materials have some penetration of light, which is why you can use Fresnel shaders and SSS shaders on anything. Fortunately, you can decide when to say when, and concentrate only on those materials that really need to react to light with the proper IOR.