# Homework Help: Total internal reflection inside a fiber optic cable

1. Jul 16, 2013

### aChordate

1. The problem statement, all variables and given/known data

A multi-mode fiber optic cable has a core diameter d = 115 mm, ncore = 1.42, and ncladding = 1.17. What is the largest possible angle θin relative to the surface normal that light can enter the fiber (i.e. pass from air into the fiber core) and experience total internal reflection inside the fiber?

2. Relevant equations

sinθc=n2/n1

3. The attempt at a solution
d = 115 mm
ncore = 1.42
θin=?

Do I have the correct equations?

2. Jul 17, 2013

### PeterO

Is the end of this optical fibre squared off , like a cylinder, or basically hemispherical, like many of these fires are when created?

Also I like to match subscripts, to avoid losing track of which angle you are calculating, so I would write either:

or

n1sinθ1=n2sinθ2

Having established [written down] that medium 1 was the cladding and material 2 was the core.

(and you seem to have left the sin out - though if the angles involved are small you can approximate sinθ to θ

3. Jul 17, 2013

### aChordate

How do I find the angles?

4. Jul 17, 2013

### PeterO

For T.I.R. the angle of refraction is set at 90o so one of those angles is 90o.

Which one do you think that might be.

Hint: if you pick the wrong one, the problem gives no real solution.