Refraction is often found in conjunction with reflective surfaces, think of a window that you can both see through and has a reflection, so it is most likely the case that you will add both a reflection layer and a refraction layer to your material. It’s just something that needs to be kept in mind, especially when working with large scenes, tight deadlines, slow machines, and impatient clients/bosses. It now must not only calculate the lighting and material, but also the way in which the light/materials reflect or refract and what will be seen as a result. As a side note, you will notice that adding a reflection layer and/or a refraction layer will cause an increase in render time because of the increase in computing the machine is asked to do. Otherwise, the computer will try to calculate this difference, but be unable to (think of it as dividing by 0) and freak out, resulting in a black output. As a result, any object that you apply a material with a refraction layer to MUST have a thickness. The light waves are deflected or change angle between the top surface of the material and the back surface.

But to review, refraction is the visual distortion that occurs as a result of a transparent or translucent material having some level of density to it.

For those who aren’t keeping up, I have gone through a brief explanation of the differences between reflection and refraction in my previous post.

Vray rhino materials glass