Thin - Thick
Thick or Thin is a setting for transparent materials, or transparent objects, which causes them to be rendered as a solid piece of glass, (Thick), a thin piece of glass, (Thin).
If the object is not space-enclosing and is transparent (for example, a pane of glass modeled with a single surface), select Thin. Thin means that the surface will be treated as a two-sided object for refraction purposes (like a sheet of glass). This is important if you are modeling a sheet of glass with a single surface.
If the surfaces are part of a solid object whose surface normals consistently point outward, select Thick. This means that each surface will be treated as single sided.
In the example on the right, the orange "bagels" are inside the boxes, the purple ones are behind, the left box is set to Thick, and the right box is set to Thin. The box on the left refracts as if it were a solid piece of glass. The one on the right treats each edge of the box as single pane of glass and does not provide refraction.
Transparent materials and glass will be rendered as Thin, as the default, unless the transparent material is marked as thick in the Material Wizard , or the face or object containing the glass is marked Thick as an Object Property .
Thick should be used when you use two faces for a pane of glass (or 2 or 4 faces for a double glazed windows), you will have to mark the faces as "Thick". You can do this when making the transparent material, or using "Object Properties" on the glass.