sounds a lot less complicated than “Index of Refraction”, so I’ll go with that as a title for this post.
When light passes through a transparent medium (water, glass, diamond etc.) it gets bent. Just how much the light bends is measured in what is called an IOR or index of refraction. Rendering engines (a.k.a. programs) use this value to simulate a wide range of materials. You can start with a standard glass material in most renderers and change the IOR value to create gemstones, water or even air. The latter not being all that impressive as you’ll see below.
Here are three transparent spheres sitting on a checkered plane for comparison. The first is set to an IOR of 2.45 which is the same as diamond.
The second has an IOR of 1.55 which corresponds to Glass.
Finally, a transparent sphere set to an IOR of 1 is shown below. This is the same as air but I promise there is a sphere here.
You can do a quick search online for the IOR of any material you wish to render. For instance water is 1.33. Congratulations, you are now more of a geek than when you began reading this post.