# Glass and water

A capillary tube immersed in water (refractive index of 1.33) is made of glass with an index of refraction 1.55. The outer radius of the tube is $$2.5\text{ mm}$$. The tube is filled with a liquid with the index of refraction 1.45. What should the minimum internal radius of the tube be (in $$\text{mm}$$) so that any ray that hits the tube would enter the liquid in the capillary?

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