# What is the internal radius of the optical fibre

• Ajaroy
In summary, the conversation discusses the determination of the minimum internal radius of a capillary tube made of glass with refractive index n1, filled with a liquid of refractive index n2<n1. The question is how small the inner radius can be so that all rays incident on the outside enter the inner tube. The conversation also suggests drawing a diagram and considering the possibility that it is not related to an optical fiber.

## Homework Statement

A Capillary tube is made of glass of refractive index n1 . The outer radius of the tube is R.The tube is filled with a liquid of refractive index n2 < n1 .what should be the minimum internal radius of the tube so that any ray that hits the tube would enter the liquid

## The Attempt at a Solution

I know that in an optical fibre they are core and cladding.it operates by principle of total internal reflection.i have calculated maximum incident angle possible
As i = arcsin √n12
-n22

How to get terms in terms of R and should we consider any size approximations as it is a capillary tube is what I can't understand.

I don't think this question is related to an optical fiber with cladding. Consider the drawing that shows a ray coming in from the outside. A typical ray is shown incident on the capillary. The question is how small the inner radius can be so that all the rays incident on the outside enter the inner tube. Draw a neat ray diagram of the path that the ray shown follows and write some equations down. Hint: A useful parameter is the vertical distance between the ray and the dotted line.

#### Attachments

• Capillary.png
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Ajaroy said:
A Capillary tube is made of glass of refractive index n1 . The outer radius of the tube is R.The tube is filled with a liquid of refractive index n2 < n1 .
what should be the minimum internal radius of the tube so that any ray that hits the tube would enter the liquid
Have you considered the possibility that this is not about an optic fibre? It sounds to me as if the light might be striking perpendicular to the axis of the tube. Otherwise I can't see any reason why a paraxial ray should ever enter the liquid.

Try drawing a diagram.