This is all really helpful, thank you. I think I'm making progress. I still have
a few issues.

This is what I now have:

global_settings{photons{spacing 0.1}}

#declare my_texture =
 texture {
  finish{
     reflection{
      0.3, 1
      fresnel
    metallic
   }
   specular 1
   roughness 0.1
  }
 }

background { color Blue }
camera {
    location <0, 10, -50>
    look_at  <0, 0,  20>

  }

difference{
  box{< -10,  0,  0>,<10,  20,   20>}//outside of the box
  box{<-9.6, 0.4, 0.4>,<9.6, 19.6, 19.6>}//inside of the box
// the walls now have a thickness of 0.4 unit
  box{<-9,1, 0>,<9, 10, 0.4>}// the opening
  texture{
   object{
    box{<-9,1,0.0001>,<9,10,1 >}
    my_texture,
    pigment {rgbt 1}
  }
  }


photons{target reflection on}

 rotate y*20
}

light_source { <5, 20, -30>
 color White
}

I'm getting an error on the

texture{
   object{
    box{<-9,1,0.0001>,<9,10,1 >}
    my_texture,
    pigment {rgbt 1}
  }
  }

code, saying that there is a missing }, but I don't see it.

Also, did I put the photons{target reflection on} in the right place, or do I
need to create an object that includes the box and the photons?

Rhonda

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