"Kenneth"  wrote in message
<web.491639ac12932d0b78dcad930@news.povray.org>:
>						    It would be interesting to
> know just what this 'composite IOR' is (if that's the right word for it.)

There is no equivalent "composite IOR". What happens is that the light ray
should have been bent inside the object but was not.

You may be able to interpret things in terms of real world physics, but that
will just give something awkward. Right now, I see two ways:

- Imagine there is a boundary between the two IORs, but that that boundary
  is always orthogonal to the light ray, thus not bending it. I am not
  really sure that the condition actually defines a surface, and it would
  probably not fit the border condition anyways.

- Imagine that the outside part of the object has a different IOR. That can
  account perfectly for the light rays that pass through the object, but
  that brings back the problem to the light rays that do not.

There is the same problem for light rays between a light source and an
object: if they go through a transparent object with IOR, the IOR is
ignored.

The bottom line is that ray tracing can not perfectly simulate real world
physics.

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