I am finding this whole thread pretty interesting and it is something which i
have looked into before. Having played around with these features and building
a setup which has the technicalities of a rainbow taken into account I created
the following image a while back.

Notably this is not quite what you are looking for, the light source comes
through the plane (by means of the blue hole) and the rainbow emerges due to
bending the light back (This may seem a strange idea however if you imagine
that the screen is the observer then by looking at the screen we see the light
that would be seen by the observer).

The set up is similar to what you are trying. This thread has prompted me to
play around with the set up and I have found that oddly enough the setup does
require that caustics be on, this doesnt make sense but appears to be the case.
I will be continuing to play around so will let everyone know what I find with
further investigation.

Malcolm

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