Thank you for your reply.
>May I know the fluent version you are using.
Yes! I used ver. 2021 R2.
I studied these parts before. My case may be different.
In my model, the mediums a and b are the same. As 5.3.6.7.1 described, it seems that the refractive index 
 and  
were important to change.
And if the wall is 0 thickness, the refractive index is not important.
If the wall has thickness, the absorption coefficient can be set to simulate raditation transmission and absorption.
But my case, the wall is 0 thickness, and medium a , b is the same. So how to do it?
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