Question

An optical fibre uses flint glass clad with crown glass. What is the critical angle?

Short answer

The critical angle is $66.3^{°}$.

Step-by-step solution

Definition of critical angle

The critical angle is the angle of incidence that produces a 90-degree angle of refraction.

Critical angle of fibre glass

Let us consider the given problem.

The expression to find the critical angle:

${\theta }_{\circ }={\sin }^{-1}\left(\frac{n_2}{n_1}\right)........\left(1\right)$

Here,

${\theta }_{\circ }$Is critical angle

$n_1$Is index of refraction of medium

$n_2$Is index of refraction of medium 2

Substituting 1.52 for n₁ and 1.66 for n₂ by in equation (1),

$$\begin{gathered} {\theta }_{\circ }={\sin }^{-1}\left(\frac{1.52}{1.66}\right) \\ =66.3^{°} \end{gathered}$$

Therefore, when optical fibre is encased in flint with crown glass, the critical angle is $66.3^{°}$.