Question

A step index fibre has a relative refractive index of 0.88%. What is the critical angle at the core cladding interface?

• A. (a) 60°
• A. (b) 75°
• A. (c) 45°
• A. (d) None of these

Answer 1

(d)

Sol. Here $\frac{n_{1} - n_{2}}{n_{1}} = \frac{0.88}{100}$ ⇒ $\frac{n_{2}}{n_{1}} = 0.9912$

∴ Critical angle

$\theta_{c} = \sin^{- 1}\left( \frac{n_{2}}{n_{1}} \right) = \sin^{- 1}(0.9912) = 84{^\circ}24'$