What is the real radius of capillary tube?

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You have a capillary tube in a capsule whose refractive index is $n = 2$. You notice that the radius of capillary tube seems $1.50mm$. What is the real radius of capillary tube?

I'm having trouble with this optics question.

Let's take a look at what we have:

Refractive index of capsule: $n = 2$

Radius of capillary tube: $1.50mm$

Refractive index of air: $n = 1$

Can I get your thinkings?