Refractive Index

Specifies the Index of Refraction for the selected faces.

Refraction is only calculated if the Trace Reflections and Trace Glass option on turned on in the Trace Tab page of the Render dialog.

Refraction is the bending of light as it passes through a transparent surface. This quality is responsible for the optical properties of a lens or magnifying glass. Refraction is also apparent when you look at a room through a glass full of water. Light is bent once as it passes through one side of the glass, again as it enters and exits the water, and again as it passes through the far wall of the glass.

The amount that light bends as it passes through any given material is defined by a standard number called the "Index of Refraction."